Understanding Wood Finishing Hardcover E-Book

Bob Flexner

• Herausgeber: Fox Chapel Publishing
• Kategorie: Lebensstil
• Sprache: Englisch
• Veröffentlichungsjahr: 2016

Seitenzahl: 701

Beschreibung

For more than 18 years, Bob Flexner has been inspiring woodworkers with his writings and teachings on wood finishing. Now, from this best-selling author comes the long-awaited and completely updated second edition of UNDERSTANDING WOOD FINISHING-the most practical, comprehensive book on finishing ever published. The first edition of UNDERSTANDING WOOD FINISHING has sold hundreds of thousands of copies and is widely regarded as the bible of wood finishing. "We use UNDERSTANDING WOOD FINISHING as the textbook for our students training to go into the furniture industry," says David Miles, wood technology professor at Pittsburg State University. "It's the best written, most accurate, and most thorough wood finishing book in print-by far."

Leseprobe

Contents

Introduction

Safety

Chapter 1: Why Finish Wood, Anyway?

■ Sanitation

■ Stabilization

■ Decoration

Chapter 2: Preparing the Wood Surface

■ Selecting the Lumber

■ Sanding and Smoothing

■ Sandpaper

■ Dewhiskering

■ Glue Splotches

■ Dents, Gouges, and Holes

■ Wood Putties

Chapter 3: Tools for Applying Finishes

■ Making a Rubbing Pad

■ Rags

■ Brushes

■ Using Brushes

■ Common Brushing Problems

■ Spray Guns and Equipment

■ How a Typical Spray Gun Works

■ Spray Booths

■ Common Spraying Problems

■ Aerosol Spray Finishing

■ Compressors

■ Common Spray-Gun Problems

Chapter 4: Staining Wood

■ Understanding Stains

■ What Makes a Stain

■ A Guide to Stains

■ Chemical Stains

■ Bleaching Wood

■ Ebonizing Wood

■ Using Aniline Dyes

■ Matching Color

■ Dyes and Fade Resistance

■ Applying Stain

■ Washcoating Before Applying Stain

■ Washcoats

■ Staining End Grain

■ Common Staining Problems, Causes, and Solutions

Chapter 5: Oil Finishes

■ Our Ancestors and Linseed Oil

■ Applying Oil and Oil/Varnish Finishes

■ Oil Finishes and Penetration

■ Understanding Oil

■ The Food-Safe Myth

■ Understanding Varnish

■ Which Is Which?

■ Bleeding Oil Finishes

■ How to Tell Which Finish You Have

■ Maintenance and Repair

■ Guide to “Oil” Finishes

Chapter 6: Wax Finishes

■ Using Wax as a Finish

■ How to Make Your Own Paste Wax

■ Applying Paste Wax

■ Compatibility with Other Finishes

Chapter 7: Filling the Pores

■ Filling Pores with the Finish

■ Filling Pores with Paste Wood Filler

■ Finish vs. Paste Wood Filler

■ Oil-Based vs. Water-Based

■ Common Problems Using Oil-Based Paste Wood Filler

■ Using Oil-Based Paste Wood Filler

■ Using Water-Based Paste Wood Filler

Chapter 8: Introduction to Film Finishes

■ What’s in a Name?

■ Making Sense of Finishes

■ How Finishes Cure

■ Solids Content and Mil Thickness

■ Controlling Sheen with Flatting Agent

■ Finishing Materials: How They Cure

■ Comparing Evaporative, Reactive, and Coalescing Finishes

■ Classifying Finishes

■ Sealers and Sealing Wood

■ Solvents and Thinners

■ Finish Compatibility

■ The Future of Film Finishes

Chapter 9: Shellac

■ Pros & Cons

■ What Is Shellac?

■ Categories of Shellac

■ Alcohol

■ Shellac in Modern Use

■ Brushing and Spraying Shellac

■ French Polishing

■ Common Problems Applying Shellac

■ Padding Lacquer

Chapter 10: Lacquer

■ Nitrocellulose Lacquer Variations

■ Pros & Cons

■ Advantages of Lacquer

■ Lacquer Thinner

■ Comparing the Solvents in Lacquer Thinner

■ Crackle Lacquer

■ Common Problems Applying Lacquer

■ Fish Eye and Silicone

■ Spraying Lacquer

■ Lacquer’s Problems

Chapter 11: Varnish

■ The Mix of Oil and Resin

■ Pros & Cons

■ Characteristics of Varnish

■ Applying Varnish

■ Identifying Varnish Types

■ Wiping Varnish

■ Brushing Varnish

■ Turpentine and Petroleum-Distillate Solvents

■ Common Problems Applying Varnish

■ Gel Varnish

■ The Future of Varnish

Chapter 12: Two-Part Finishes

■ KCMA Testing Standards

■ Catalyzed Finishes

■ Pros & Cons

■ Two-Part Polyurethane

■ Crosslinking Water Base

■ Epoxy Resin

Chapter 13: Water-Based Finishes

■ What Is Water-Based Finish?

■ Pros & Cons

■ Characteristics of Water Base

■ Common Problems Applying Water Base

■ Brushing and Spraying Water-Based Finishes

■ Glycol Ether

■ Is Water Base for You?

Chapter 14: Choosing a Finish

■ Appearance

■ Protection

■ Durability

■ Ease of Application

■ Safety

■ Disposing of Solvent Waste

■ Reversibility

■ Rubbing Qualities

■ Comparing Finishes

■ Guide to Choosing a Finish

■ How to Choose

Chapter 15: Advanced Coloring Techniques

■ Factory Finishing

■ Glazing

■ Antiquing Wood

■ Toning

■ Pickling

■ Step Panel

■ Common Glazing and Toning Problems

Chapter 16: Finishing the Finish

■ Cutting Through

■ Additional Factors in Rubbing a Finish

■ Synthetic Steel Wool

■ Rubbing with Steel Wool

■ Comparison of Rubbing Lubricants

■ Leveling and Rubbing

■ Machine Rubbing

Chapter 17: Finishing Different Woods

■ Pine

■ Oak

■ Walnut

■ Mahogany

■ Hard Maple

■ Cherry

■ Ash, Elm, and Chestnut

■ Aromatic Red Cedar

■ Soft Maple, Gum, and Poplar

■ Birch

■ Oily Woods

Chapter 18: Caring for the Finish

■ Causes of Finish Deterioration

■ Applying Liquid Furniture Polish

■ Preventing Finish Deterioration

■ Deterioration Causes and Preventions

■ How to Choose

■ Furniture Polish Overview

■ Types of Furniture-Care Products

■ Caring for Antique Furniture

Chapter 19: Repairing Finishes

■ Removing Foreign Material

■ Repairing Superficial Damage

■ Repairing Color Damage in the Finish

■ Touching Up the Color

■ Filling with a Burn-In Stick

■ Filling with Epoxy

■ Repairing Deep Scratches and Gouges

Chapter 20: Exterior Finishes

■ Wood Degradation

■ Slowing Degradation

■ Finishing Exterior Doors

■ Staining Wood Decks

■ UV Protection

■ How to Choose

Chapter 21: Removing Finishes

■ Stripping Solvents and Chemicals

■ Quick Stripper Identification

■ Using Oxalic Acid

■ Stripper Safety

■ Breaking the Code—An Overview of Strippers

■ Professional Stripping

■ Common Problems Using Strippers

■ Deterioration of Finishes and Antiques Roadshow

■ Using Strippers

■ Which Stripper to Use

Afterword

Sources of Supply

Index

Introduction

I learned wood finishing in the mid-1970s from a couple of refinishers with a business down the street from my woodworking shop. Looking back, I realize that I became a pretty good finisher. I learned to spray lacquer and catalyzed finishes, use dyes, glazes, and toners, fill pores, and rub out finishes.

But I was simultaneously reading the woodworking magazines at the time, and gradually I lost my confidence. The more I read about finishing, the more confused I became—until by the early 1980s I stopped doing finishing altogether and began farming it out to others.

This continued for a few years, but I wasn’t satisfied because I no longer had control. I was at the mercy of others, and I started getting complaints from some of my clients. I became very frustrated.

The dilemma I found myself in didn’t make sense. Finishing couldn’t be that hard. I had to figure out some way to make sense of it. So I started checking out finishing and refinishing books from the public library, and I redoubled my efforts to find information by reading woodworking magazines. No help. I just became more confused. It seemed that every time I thought I understood some explanation or procedure, I then read something that contradicted it.

The Breakthrough

One day I called my friend Jim, who had a good background in chemistry, to ask if he could explain why alcohol would separate furniture joints that had been glued together with animal hide glue.

“What’s hide glue?” he asked. “It’s glue that’s made from animal skins,” I explained. “Oh, protein!” he exclaimed. And he proceeded to tell me all about hide glue—all the information I had been unable to find in any of the woodworking books I’d searched through—how it worked, how and why it deteriorated, why it bonded without clamps, why alcohol would crystallize it and steam would dissolve it, and so on.

I was dumbfounded. For years I had been looking in vain for accurate information on the hide glue I used for restoring antique furniture. And Jim had known it all along—simply because he understood the chemistry of protein.

That conversation ended with Jim offering to take me over to the local university engineering library to see what we could dig up. I walked out with an armload of books dealing with hide glue.

Nearby the books on glues were several shelves of books on finishes, including books on the chemistry and technology of stains, dyes, solvents, oil, and wax. Several weeks later I went back and checked out a few of these books.

I don’t have a background in chemistry or engineering, so I found these books difficult to understand at first. To further my education, I joined the national association of paint and finish chemists and attended some seminars and conventions. I spent countless hours talking to the chemists who actually make the raw materials used in our finishing products. I found these chemists to be much like the majority of woodworkers I’ve met: They love to share what they know with anyone who shows interest.

Slowly I began making sense of finishes. I found it remarkable how understanding each product and how it worked made its application obvious, and how this understanding helped me solve problems when they occurred in my shop. No longer did I have to learn everything by trial and error, a method that is hugely expensive and frustrating and one that requires a long-term commitment to succeed. (If you’re not doing finishing on a regular basis, I don’t see how you can ever get very good at it by trial and error alone.)

I also found it hard to understand why no one had taken this information and put it in a form that would be useful to woodworkers and finishers. No one had tried to bridge the chasm that divides finish chemists, who understand quite well what’s true and not true about finishes, from those of us who use finishes. So I decided to do it myself with the first edition of this book, which was published in 1994.

The Half-Right Rule

The book was extremely successful—beyond my wildest dreams. But it didn’t solve the problem. Published information on finishing is still confusing and contradictory. We are all still encumbered by what I call the “half-right rule.” Half of what we read or hear about finishing is right. We just don’t know which half!

Why is this? Why does the information about wood finishing continue to be so inadequate? Wood finishing is, after all, a very simple craft. It involves little more than transferring a liquid from a container to the wood using a rag, brush, or spray gun. Each of these tools is easy to master. (By way of comparison, think of all the tools you have to master to be good at woodworking.)

I believe there are two explanations. The first is that finishes are chemistry—they are molecules of various sorts put together to make a liquid with specific characteristics. So unlike woodworking and woodworking tools (which are physics), you can’t see differences. You can’t see the difference between varnish and lacquer, for example, not in the can, not even on the wood. In contrast, you know right away that a dovetail isn’t a mortise-and-tenon and that a band saw isn’t a table saw (even though it also has a table).

Not being able to see differences makes the second explanation possible and probably inevitable. It is that manufacturers of finishing products have a lot of room to mislead and exaggerate in their product naming and promotion. This results in your being misled and also in those people writing about finishing not knowing what is correct and what isn’t. So there’s no effective check on a manufacturer’s natural inclination to make its products appear better and sometimes even different than they actually are.

I am convinced that the understanding of finishing would be far less a chore (and this book much smaller) if there weren’t so much misinformation to debunk. You may well find that it will take more effort to unlearn what is wrong than to learn what is right.

The Secret Is That There Is No Secret

The reticence of manufacturers to provide good information is perhaps most evident in their reluctance to tell us what ingredients they are using. The excuse is usually that the information is proprietary. The manufacturer doesn’t want to give away its “secrets.”

At one time, manufacturers, and even the individual finishers and painters who made their own finishes, may have had secrets. But finish formulation has become a sophisticated science in the last 100 years. The essential chemistry of almost all finishing products has been thoroughly understood for decades.

There is very little that is new in wood finishes. And what is new has almost always been developed by the large chemical companies that produce the raw materials. These companies make their new information available to anyone who wants it, especially to the finish manufacturers who are potential customers. These raw materials’ suppliers even provide the formulations for using their products. All the finish manufacturers have to do is mix the ingredients.

So all of the manufacturers of stains and finishes (as well as you and I) have access to the same information about raw materials and making finishes. Where this information doesn’t flow is between the companies that put these raw materials into products and us, the users of the products.

Even among the companies themselves there’s no secrecy because every large finish manufacturer has access to equipment that will analyze the content of their competitors’ products. Every manufacturer can find out what every other manufacturer is selling. One of the pioneers of modern finishes William Krumbhaar, once said, “The real reason for secrecy is the necessity of concealing the fact that there is nothing to conceal.”

Modern wood-finish suppliers, in fact, are little more than marketing companies following the best business practices for selling the maximum amount of product. With all the consolidation and cost cutting that has occurred during the last several decades, there are few, if any, people left at these companies who understand much about the products being sold. This explains why you get so little useful help when you call and why the directions on labels are often inaccurate.

Toward Understanding

So how do you overcome the dual realities that you can’t master finishing by trial and error unless you practice every day, and you can’t rely on what you are being told by the manufacturers or even in third-party books and pamphlets?

It’s my experience that you do it by learning what the products are, how they work, and what they can be expected to accomplish. You don’t need to go back to the original chemistry to do this because I’ve already done it for you. And this second edition benefits from a decade of refining and adding to the information I first presented.

My hope is that the information presented in this book will make it possible for you to succeed at finishing. I also hope that others will pick up where I have left off and develop the information further. There are, after all, an infinite number of ways to apply finishing products and an infinite number of effects. (There’s only one accurate way to define the products, however.)

Above all, I hope manufacturers will begin to help us learn about their products. They can start by labeling their containers accurately and listing the ingredients on their labels. Nothing could go further to make wood finishing the easy craft it should be.

How to Use This Book

Books on finishing are difficult to read straight through because those parts you have no experience with are often difficult to follow. The information in this book is arranged linearly, in the order you actually apply a finish. But you needn’t read the early parts to understand the later. A craft is not learned linearly. It’s learned gradually, the conceptual understandings periodically buttressed by hands-on experience.

Turn to the sections of the book you’re interested in. Try some of the finishes on scrap wood before you use them on a completed project. (You wouldn’t attempt to cut your first dovetail joint on the actual drawer!)

As your skills improve, your interests will change, and you can tackle other sections. You’ll find that all the materials and techniques used in finishing are interrelated. The more you learn about one subject, the better you’ll be able to understand another.

Safety

Throughout the book I point out safety precautions you should take when using various individual finishing materials. Here is an overview.

Most of the materials used in finishing are bad for you. Solvents, such as mineral spirits, naphtha, and lacquer thinner, can cause dermatitis, dizziness, headaches, and nausea. Chemicals, such as lye, oxalic acid, and chlorine bleach, can cause respiratory and skin problems. Even the so-called “safe” strippers and water-based finishes contain solvents that are bad for your health if you breathe too much of them.

You should always protect yourself by arranging cross ventilation in your work area to provide a constant source of clean air. When you can’t ensure a flow of clean air, wear a NIOSH-approved, organic-vapor, respirator mask. (NIOSH is the National Institute for Occupational Safety and Health. It is a federal agency that does research on matters concerning worker’s health and tests and certifies respiratory protection.) In addition, you should wear gloves to protect your hands when they come in contact with the finishing materials.

But, in spite of these warnings, don’t be afraid of using finishing materials any more than you are afraid of using woodworking tools. This is an important point, because it’s becoming more and more common to read dire warnings about the hazards of using certain finishing products. Some of the warnings come from manufacturers of competing products. Some come from writers repeating the most extreme warnings they’ve heard without researching their validity. In some cases, if you didn’t know better, you could expect almost certain death from products you can buy at any paint store.

Rely on common sense when using finishing materials just as you should when using power woodworking tools. Pay attention to your body. If you start to feel lightheaded or begin coughing, or if your hands become dry and chapped, protect yourself better. Since the risk of health problems caused by solvents and chemicals used in finishing increases with exposure (your body becomes sensitized), take greater precautions if you work with these products on a regular basis.

CHAPTER 1

Why Finish Wood,Anyway?

In Brief

■Sanitation

■Stabilization

■Decoration

Why do we finish wood? It’s an extra step, or steps, that most woodworkers don’t find at all enjoyable. It’s smelly and messy, and all sorts of things can go wrong. In addition, most woods look pretty good unfinished. Why bother? There are three good reasons for finishing wood: to help keep it clean, to help stabilize it, and to decorate it.

Sanitation

Wood is a porous material. It contains countless holes of various sizes. These holes can accumulate dirt and grime from handling, atmospheric contaminants, and food. Grimy wood is unattractive, and it can be a health hazard, providing a breeding place for bacteria. A finish seals the porous surface, making it less susceptible to soiling and easier to clean.

Stabilization

Besides being porous, wood is hygroscopic: It absorbs and releases moisture. Moisture within wood is called moisture content; moisture in the environment is either liquid water or water vapor (humidity). Wood responds to changes in the level of moisture around it. If you put very dry wood in water or in an area of high humidity, the wood will absorb moisture and swell. If you put wood that has a high moisture content in a relatively dry climate, the wood will release moisture and shrink.

These dimensional changes, commonly called wood movement do not occur consistently throughout a piece of wood. The surface of wood, for instance, responds more readily than the core. Wood swells and shrinks mainly across the grain; that is, in the width and thickness of boards, not appreciably in the length. And wood swells and shrinks more around the annular growth rings than it does perpendicular to the rings. The result of these different responses is that wood movement generates great stresses in wood and on the joints that hold pieces of wood together. The stresses cause splitting, checking, warping, and weakening of the joints. A finish slows moisture exchange, thus reducing the stresses and stabilizing the wood.

As a general rule, the thicker the coating of finish, the better it limits moisture exchange. This exchange does not have to be in the form of liquid water. It can be, and usually is, water vapor. Water-vapor exchange causes much damage to otherwise sheltered wood furniture and woodwork. It just does this more slowly than in wood subjected to liquid-water exchange.

MYTH

Finishing both sides of wood prevents, or at least reduces, warping.

FACT

A finish will slow but will not prevent or reduce warping caused by wood shrinking more tangentially (around its growth rings) than radially (perpendicular to its growth rings). Moisture vapor will still work its way through the finish and cause the same degree of warping. Warping caused by compression shrinkage on the side more exposed to wetting and drying (usually the top side) won’t be hindered at all. This is the reason tabletops exposed to spills and water from wiping with damp cloths should be refinished whenever the finish begins to deteriorate or wear badly.

Figure 1-1: When dry wood is exposed to moisture, the cell walls swell. If the wood can’t expand, the cells compress.

Splits, Checks, and Warps

To understand better how moisture exchange causes splits, checks, and warps, look at Figure 1-1. A solid piece of kiln-dried wood is clamped securely so that it can’t expand in width. Then it is exposed to 100 percent humidity for a period of time. The cell walls swell and the wood tries to expand, but it is constrained by the clamps. So the cell walls compress, changing from cylindrical to oval in shape.

If the board is released from the clamps and the humidity dropped to 30 percent, water evaporates and the cell walls shrink. But the cells don’t return to their cylindrical shape: They remain flattened. So the board shrinks, becoming narrower than it was originally. If the board is reclamped and exposed again to high and then low humidity, it will shrink further. This phenomenon is called compression shrinkage (also compression set). It explains why nails and screws work loose in wood, and why the wooden handles of hammers and hatchets loosen over time, after continually absorbing and releasing moisture.

Compression shrinkage also accounts for splits developing in the ends of a board, checks in the middle of a board, and cupping (a type of warp) on the side of a board exposed to the most water (Photos 1-1 through 1-3). In each case part of the board comes in contact with water and tries to expand more than the rest of the board will allow it to. After a number of cycles of restricted Wood cell, normal circular shape expansion followed by full contraction, that part of the board changes shape or splits. Of course, these types of problems are less likely to occur if the finish is kept in good shape. Less water will be able to get to the wood.

TIP

Understanding the concept of compression shrinkage leads to the counterintuitive, but effective, method for correcting warps caused by repeated exposure to water on one side. Hold the board in clamps so it can’t expand; then wet the convex side (usually the bottom side of tabletops) many times, letting it dry after each. The convex side will be compressed and shrink, bringing the board flat.

Photo 1-1: When wood absorbs moisture, more is absorbed near the end than in the middle because the end grain is more porous than the other surfaces and because the end grain together with the nearby face grain presents more surface area. As a result, the end of a board is prone to expand more than the middle of a board. But the middle acts like a clamp on the end, causing compression shrinkage. After a number of cycles, the end of the board splits to relieve the stresses. You see this type of compression shrinkage on the ends of any boards exposed to repeated contact with water.

Photo 1-2: When moisture comes in contact with just part of a board, the wood cells in that part swell. But the surrounding wood acts like a clamp and prevents the swelling. This causes compression shrinkage and the resulting checks. You see this type of compression shrinkage on parts of tabletops subjected to repeated contact with water, as from a leaking potted plant.

Joint Failure

Joint failure also is accelerated by excessive moisture exchange—usually in the form of water vapor, not liquid water. The cells in wood are like soda straws running lengthwise in boards. The cell walls swell and shrink, changing the width and thickness of boards, but not the length. When boards are joined with the grain running perpendicularly, as they inevitably are in wood structures, the swelling and shrinking in different directions put great stress on the joints. As glue ages and loses its flexibility, the contrary movement in any cross-grain construction causes joint failure. This is why glued furniture comes apart in time, and why it makes no sense to claim that any glue will hold joints together forever (Figure 1-2).

The speed at which moisture exchange damages wood or breaks the glue bond in joints varies depending on the environmental conditions. Wood or furniture left outside in the weather develops splits, checks, warps, and joint failure much sooner than wood or furniture stored under cover. Wood or furniture stored under cover develops problems much faster than wood or furniture stored in a controlled environment (such as inside your house). Furniture moved from a damp climate such as New Orleans to a dry climate such as Phoenix often develops joint problems within a year or two even if the wood is finished. The best environment for storing wood or wooden objects is one of constant temperature and humidity. This is the environment museums try to maintain.

Photo 1-3: When wood is exposed to more water on one side than the other, the imbalance causes cupping. The side exposed to the most moisture has its expansion restricted by the thickness of the board, resulting in compression shrinkage. You see this type of compression shrinkage on decks and tabletops, where the cupping is almost always on the top side of the boards. This is so even though the direction of the rings would often indicate the opposite should happen, and even though the top may have been finished and the bottom not. Keep in mind that finishes on tabletops age and become worn, thus losing their resistance to water penetration. And it is the top that gets wiped with a damp or wet cloth after meals.

Figure 1-2: Wood shrinks and expands across the grain, not along the grain. When boards are joined with the grain running perpendicularly, the contrary shrinkage and expansion eventually causes joint failure.

Figure 1-3: This chart shows how effectively a finish in good shape stabilizes the moisture content of wood through seasonal changes in humidity. Inhibiting moisture-vapor exchange effectively minimizes the stresses in wood that can be caused by wide swings in humidity.

A finish slows moisture exchange no matter what temperature or humidity conditions surround the wood. A finish makes the wood, or the object made of wood, last longer, and this is the problem with the “Do not refinish” message being conveyed by popular antique-valuation television shows, such as Antiques Roadshow. If many people heed this message, it will lead long-term to the destruction of a great deal of furniture (Figure 1-3).

NOTE

▼

No amount of finish or paint totally stops watervapor exchange. For example, finished or painted wooden windows and doors shrink and let in cold air in the winter but swell tight in the spring and summer. A good finish reduces the extremes of variation that would otherwise occur during seasonal humidity changes, but it doesn’t stop these variations.

Decoration

In addition to stabilizing wood and protecting it from dirt and grime, finishing wood is decorative. Even if you apply nothing more than a simple oil or wax finish, you are making a decorative choice. There are an infinite number of ways you can decorate wood, but all can be grouped into three categories: color, texture, and sheen.

Photo 1-4: Two-part bleach was used to take the color out of the ash on the top of this coffee table. Black dye was used to ebonize the legs, also ash. Photo courtesy of the maker: Michael Puryear.

Color

There are four ways you can apply color to wood. If you change the color through chemical reaction, it’s called bleaching or chemical staining. If you apply a colorant directly to wood, it’s called staining. If you apply a colorant between coats of finish, it’s called glazing. If you add a colorant to the finish itself and apply it to the wood, it’s called toning or shading if you can still see the wood through the colored finish; it’s called painting if you can’t. Each of these methods produces a different decorative effect:

■ Bleaching takes the color out of the wood, leaving it almost white (Photo 1-4). Chemical stains react with chemicals natural to the wood or added to the wood to change the color.

■ Stain applied to bare wood amplifies the figure and grain of the wood. Stain also highlights problems in the wood, such as scratches, gouges, machine marks, and uneven density.

■ Glaze, applied thinly and evenly to an entire surface, changes the tone of the wood’s color and may highlight pores and recesses (Photo 1-5). Applied thickly, glaze can be manipulated with various tools to imitate wood grain, marble, or other faux (false) effects.

■ Shading, toning, and painting change the tone of the wood’s color without highlighting pores and recesses. Shading and toning allow you to see the figure and grain of the wood. Painting totally obscures the wood’s features. Shading changes the color tone only in the areas you want. Toning changes the color tone evenly over the entire surface.

A subtler, but still important, method of controlling color in wood is with the finish. Some finishes are perfectly colorless, while others add a slight orange coloring (usually referred to as yellowing). Still other finishes, such as amber shellac, add considerable orange coloring (Photo 1-6).

Photo 1-5: To accent the depth of the carving, glaze was applied to this ball-and-claw foot over the first coat of finish. The glaze was then wiped off the high areas to leave the recesses darker. Then the topcoats were applied.

Photo 1-6: This bedside cabinet of mahogany, mulberry, and Port Orford cedar was finished with oil to bring out the natural color of the wood, followed by a thin coat of shellac, which gives a warm amber glow. Photo (above and on p. 1) courtesy of the maker: Charles Radtke.

Photo 1-7: The grain in this Hawaiian koa and ebony chair was filled with paste wood filler. The finish was then leveled mirror-flat and rubbed to a satin sheen.

Texture

All woods have a natural texture dependent upon the size and distribution of the pores. You can preserve this texture by keeping a finish very thin. This thin-finish look is very popular. It’s often called a natural wood look, and it is what you get when you finish with oil or wax. You can get the same look with film finishes, such as varnish, shellac, lacquer, or water base, as long as you keep them thin. Scandinavian teak furniture is finished with a film-building finish (usually conversion varnish) that is applied thin, not with oil, as is commonly claimed and believed.

By filling, or partially filling, the pores, you can completely alter the texture of the wood. You can fill the pores with paste wood filler or with many coats of finish that you sand or scrape back (Photo 1-7). The most refined finishes (for example, those commonly used on very expensive dining tabletops) have filled pores.

Sheen

Sheen is the amount of gloss the finish has. There are two ways to control sheen. The first is by choosing a finish that has the sheen you want built into it: gloss, satin, or flat. The second is by rubbing and polishing the cured finish to the sheen you want.

CHAPTER 2

Preparingthe Wood Surface

In Brief

■Selecting the Lumber

■Sanding and Smoothing

■Sandpaper

■Dewhiskering

■Glue Splotches

■Dents, Gouges, and Holes

■Wood Putties

A quality finish is impossible to obtain if you don’t prepare the wood properly. You probably know this already. I’m sure you’ve at least heard it. Most woodworkers dread the preparation steps, skip through them, and get a poor finish as a result. Others spend more time and effort than they need to scraping, sanding, patching, sand ing, steaming out dents, sanding, and more sanding. Both extremes are probably due to a lack of understanding of what needs to be achieved.

The most glaring examples of how poor understanding leads to lower-quality work occur when the woodworker and the finisher are different people, and the communication between them is deficient. This situation is common in house construction, where cabinetmakers and trim carpenters often pay scant attention to the little things they can do to make the finisher’s job easier and of better quality. “Oh, the finishers will take care of that,” they’ll tell you.

The usual cause for overpreparation is the belief that sanding to 400-grit or finer produces better results. The wood looks better when sanded to 400-grit, after all. Why shouldn’t the finish on the wood also be improved?

When you have control of a project from beginning to end, you’ll find that it pays to begin thinking of the finish from the start. In fact, the old wisdom holds that a good finish begins with the selection of the lumber itself.

Photo 2-1: These four woods—pine, maple, mahogany, and oak—are each stained on the right side with the same stain. But they still look entirely different because the woods are different colors, and they have different figure and grain. Always choose a wood or woods that will give you the look you want at the end. In many cases, you can’t make one wood look like another.

There are four steps in preparing wood for finishing:

1

Selecting, cutting out, and shaping the lumber. Many potential finish problems can be avoided by proper attention here.

2

Sanding or smoothing the surface. This is the most unpleasant operation for most woodworkers, so knowledge of the tools and some thought about what you’re trying to achieve can go a long way toward reducing the drudgery and improving results.

3

Dealing with glue that gets on the surface of the wood. Glue will show up as light splotches through the stain and finish.

4

Correcting surface imperfections in the wood, such as dents, gouges, and splits, and filling gaps in the joints left by a less-than-successful glue-up. This step could be called “The woodworker’s eternal quest for a wood putty that takes stain.”

Selecting the Lumber

Woods of different species can vary greatly in grain pattern, and be impossible to make look the same short of painting them. For example, oak can’t be made to look like mahogany, pine like walnut, or maple like ash. You need to think of how you want the object you are building to look when it is finished, and be sure that the species you are choosing can be made to look this way (Photo 2-1).

Even within a single species, wood varies greatly in color and figure. In some species there is a pronounced difference between sapwood and heartwood. You need to pay attention to how boards look when you’re putting them next to one another in a project. The single greatest advantage you have over factories is in the attention you can devote to wood selection and arrangement.

Whether you’re choosing boards at a lumberyard or from your own inventory, look through the supply and imagine how different grain and figure patterns would look if placed in various parts of your project. Be conscious of knots, splits, checks, and other defects, and determine how you will either use them to advantage or work around them. If you’re using veneered plywood or plan to veneer the wood yourself, think of how the figure in the veneer can be used to best advantage. Above all, pay attention to color variations, such as those between heartwood and sapwood, unless you intend to paint the piece you’re making.

For a table- or chest top, lay the boards out in different groupings, flipping and turning them end for end, until you find the best arrangement. Then mark the boards so you won’t mix them up as you joint them (Figure 2-1). If you’re making the top from veneered plywood, decide what part of the 4x8 sheet you can use most advantageously. On a chest-of-drawers, give the same attention to picking the drawer fronts. When people look at what you’ve built, they won’t see the wonderful joints you’ve spent so much time and effort making. They’ll see the design, which includes your choice of boards and their positioning, and they’ll see the finish. You won’t regret the time you spend selecting and arranging your wood.

Before you begin working your lumber, check to see that your tools are sharp and your machines are adjusted properly. Dull planer, jointer, or shaper knives and worn-out router bits will leave pronounced washboardlike mill marks in your wood that will require extra effort to remove. Chipped knives will leave unsightly ridges. Poorly adjusted machinery can snipe the ends of boards, and if the cutters on your machine tools are dull enough to burn or glaze the wood, your project could be ruined altogether (Photo 2-2). Always work toward the cleanest-cut, most mark-free surface possible.

Sanding and Smoothing

Of all the steps involved in making and finishing something of wood, sanding is the most universally detested. At the same time, curiously, it’s the step that consumes the most wasted effort. There seems to be some mystique that the more you sand, the better the end product will be. But as an old finisher I knew used to say, “When you’re in the bathtub and you’re clean, get out!” Once the wood is smooth, the mill marks and other defects are gone, and the sanding scratches are fine enough so they won’t show, there’s no reason to continue sanding. You’re finished. Your goal should be to reach this end with as little work as possible.

Figure 2-1: Marking the boards you have laid out for a tabletop or chest top ensures that you won’t get the boards mixed up as you joint them. Here are two ways to do it.

TIP

It will help to remember that just about the only reason you need to sand at all is to remove the washboardlike mill marks left in the wood by planers, jointers, shapers, and to a lesser degree, routers. Before the invention of these machine tools, there was seldom any reason to sand; indeed, there was no sandpaper. Sanding is the price you pay for using machine tools to make your woodworking easier and faster.

Photo 2-2: Dull or poorly adjusted machine tools leave much more obvious marks than sharp, well-adjusted tools. Stain and finish highlight, rather than disguise, these marks.

TIP

Mill marks and other minor flaws are hard to see before stain highlights them. Then it is too late. The best way to spot these flaws before staining is to view the wood in a raking light. Hold the wood up to a light, or position a single-point light just above the plane of the wood. If you’ve never tried this, you’ll be surprised at how much you can see.

The tools used to smooth furniture parts before the introduction of machines were hand tools—bench planes, molding planes, and scrapers of several sorts. These tools are still available, of course, and can often be used very effectively to remove mill marks. In fact, for some woodworking projects, a finely planed surface can be regarded as a final surface. In some applications, the evidence of hand-plane work—ridges from the edge of the plane iron or hollows from a scrub plane—add character to a surface, evidence of a personal touch. And for any woodworker who can’t afford or is uninterested in large power-sanding equipment, a simple tool like a scraper can be a godsend.

Whichever tools you use, you’ll usually do a better job if you prepare all of the parts before assembling. You’ll be able to secure each part to your workbench, where you can see what you’re doing clearly in good light. And you’ll be able to work in a comfortable position with any tool you choose. You’ll also avoid the difficulty of trying to sand or scrape already-assembled, right-angle joints, such as stiles and rails, or legs and rails, without putting crossgrain scratches in the perpendicular pieces (Figure 2-2). (Preparing the parts before assembly is not the same, however, as finishing the parts before assembly. Though there may be cases where doing this makes some sense, it usually doesn’t.)

Turned and carved pieces shouldn’t need any additional preparation. Turnings should be sanded (if at all) while still on the lathe. Most carvings should not be sanded at all, as sanding inevitably softens the crisp lines left by the carving tools.

Table and chest tops, sides, panels, rails, door and drawer fronts, and most moldings, however, will contain mill marks that should be removed. The most efficient tool you have, besides hand planes and scrapers, to accomplish this task is sandpaper.

Sanding Basics

The trick to efficient sanding is beginning with sandpaper coarse enough to cut through the flaws you want to remove with the least amount of effort—without creating larger scratches than necessary. This holds true whether you are using a machine or sanding by hand (Figures 2-3 and 2-4). In practice, the best grit to begin with is usually 80 or 100. If the problems are so severe that 80-grit doesn’t remove them quickly, drop back to a grit that does, or use a scraper or plane. (See “Sandpaper” on page 12.)

On the other hand, if the problems can be removed with a finer-grit sandpaper, such as 120 or 150, you are wasting time and energy beginning with coarser sandpaper. (Many people begin sanding stripped wood with 100-grit sandpaper when no more than a light pass with 180- or 220-grit would be necessary to ensure that all the finish has been removed. The wood was sanded originally, after all.)

Figure 2-2: Sanding Joined Right-Angle Parts

Figure 2-3: Sanding blocks are necessary for hand-sanding flat surfaces. The ideal dimensions depend upon the size of your hand. The dimensions indicated at right are about average. If your block is made from wood, glue on a ⅛- or ¼-inch-thick piece of felt, cork (gasket cork is available from auto-parts stores), or rubber to reduce sandpaper clogging.

Figure 2-4: You get the best use out of sandpaper by tearing the 9x11-inch sheet in thirds across the width, then folding each of the thirds in half (for use with a sanding block) or in thirds again (for use with no block).

Beginning with the wrong grit is inefficient, but the most common error made in sanding is continuing to use sandpaper after it has become dull. Pay attention to what’s happening. The cutting efficiency of sandpaper deteriorates fairly rapidly. You may cut your sanding time significantly by changing sandpaper more often.

Once the flaws have been removed, sand out scratches left by coarse sandpaper using increasingly finer sandpapers until you reach a grit that produces the size scratches you want. The scratch size makes a difference in color intensity when using a stain, particularly a pigment stain (Photo 2-3 on p. 13). The best grit to end with is usually 150, 180, or 220. I usually stop at 180-grit. The goal is to produce a surface that does not show machine marks or sanding scratches after you apply a stain or finish. If you can make the scratch pattern even, you may achieve satisfactory results sanding only to 120- or 150-grit. Stationary sanding machines accomplish this best.

If you are using a vibrating or random-orbit sander, it’s good practice to finish by hand-sanding in the direction of the grain, using the finest-grit sandpaper you used with the machine. This will remove the squigglies.

If you could sand just the right amount with each sandpaper grit, it would be most efficient to go through each consecutive grit—80, 100, 120, 150, 180. But most of us sand more than necessary with each grit, so we find that we actually spend less effort if we skip grits. This is especially the case when using sanding machines.

But sanding is very personal. We apply different pressures, use sandpapers to different degrees of wear, and sand for various lengths of time. The only way to know for sure that you have sanded enough is to apply a stain and see if any machine marks or sanding scratches show. It’s therefore wise to practice on some scrap wood until you get a feel for what works best for you.

Sandpaper

If you count all the different sandpapers available for portable and stationary sanding tools, there are now enough types on the market to fill a book of explanations. Here are the three most important facts you need to know about sandpaper.

SORTING SANDPAPER BY COLOR

Sheet sandpaper, the type you tear into smaller pieces and use to sand by hand, is easiest to understand by color (photo below).

■ Orange sandpaper is made with garnet abrasive and is available up to 280-grit. It is inexpensive and meant for sanding wood.

■Tan sandpaper is made with aluminum-oxide abrasive and is available up to 280-grit. It is more expensive than garnet, but lasts longer. It is meant for sanding wood.

■Black (wet/dry) sandpaper is made with silicon-carbide abrasive and a waterproof adhesive and is available up to 2500-grit. It is meant for sanding finishes using a water or oil lubricant.

■Gray and gold (dry-lubricated) sandpaper is made with silicon-carbide or aluminum-oxide abrasive and is available up to 600-grit. These sandpapers are coated with a dry, soaplike, zinc-stearate or similar lubricant so they don’t clog easily. They are for sanding finishes, especially sealer coats and thin finishes that don’t provide enough protection against wet lubricants. (To identify their dry-lubricated sandpaper, 3M uses the tradenames “Tri-Mite” and “Fre-Cut”; Norton uses “Adalox” and “No Fil”; Klingspor uses “Stearate.”)

There are four types of sheet sandpaper useful to woodworkers. Each is recognizable by its color: orange (garnet) sandpaper is most useful for sanding wood; tan (aluminum-oxide) sandpaper is also most useful for sanding wood (aluminum oxide is almost always used on machine sandpapers); black (wet/dry silicon-carbide) sandpaper is most useful for sanding finishes using a lubricant; gray and gold (stearated silicon carbide or aluminum oxide) sandpapers are most useful for sanding-sealer coats or thin finishes.

SANDPAPER GRADING

There are two common systems for grading sandpaper: CAMI and FEPA. CAMI (Coated Abrasives Manufacturing Institute) is the traditional American grading standard. FEPA (Federation of European Producers Association) is the European standard and is identified with a “P” in front of the grit number. These standards are fairly equivalent up to about 220-grit. Then they diverge, with the “P”-grit numbers running higher than their equivalent CAMI numbers. You don’t need to worry about the differences in these two numbering systems when sanding wood, but the differences become significant for the finer grits of wet/ dry sandpaper used to sand finishes. For example, if you intended to use 600-grit on a finish and used P600 instead, you would actually be sanding with the equivalent of 360-grit. (See “Grading Sandpapers,” below left.)

DISK BACKINGS

The most popular handheld sanding machine is the random-orbit sander, and there are two common types of sanding disks available for it: PSA (pressure-sensitive adhesive) and hook-and-loop (photo below). PSA disks are less expensive, but you can’t remove a PSA disk, use another one for a while, and then put the original back on. So PSA disks are for production situations where you are likely to wear out the disk before you would want to switch to another grit. Hook-and-loop disks work like Velcro, so you can switch disks back and forth as much as you want.

There are two types of sanding disks used with random-orbit sanders: PSA disks (top) use an adhesive backing and are less expensive; hook-and-loop disks (bottom) work like Velcro and are more expensive.

Grading Sandpapers: Most available sheet sandpaper uses the CAMI or FEPA grading system. CAMI is the traditional American grading system. FEPA is the European system and is designated with a “P” in front of the number. Up to about 220-grit, there’s not much difference in the grading systems. But above 220-grit, the grading diverges significantly, particularly in the black, wet/dry sandpapers used to level finishes.

Photo 2-3: The finer the grit you sand to, the less a stain will color the wood. This is especially true with pigment stains because there is less space in each sanding scratch for the pigment to lodge. The lighter side of this board was sanded to 400-grit, the darker side to only 150-grit.

If you are sanding by hand, always sand in the direction of the grain if this is possible (turnings and carvings are exceptions, of course), or you will surely produce cross-grain scratches that will show through the finish. It’s also wisest to move the sandpaper with the folded edge facing the direction of travel. An open edge of sandpaper is more likely to catch under a sliver of wood and lift it, which will, at the least, tear the sandpaper and could jam painfully into your hand.

No matter how fine the final grit you use, you won’t remove all of the tiny wood fibers that swell and make the wood rough to the touch if water is applied. If you intend to use a stain or finish that contains water, you may want to dewhisker the wood after your normal sanding steps. Wet the wood and resand it smooth after the water dries out. (See “Dewhiskering” below.)

MYTH

You get better results if you sand to 400 or finer grit.

FACT

Before you apply the finish, wood sanded to 400-grit will have a higher gloss than wood sanded to, say, 180-grit, because 400-grit polishes more than 180-grit. But after you apply any film finish, you won’t be able to see or feel any difference. Try it! You may save yourself countless hours of sanding in the future. See “Applying Oil and Oil/ Varnish Finishes” on p. 73 to learn how to reduce sanding with these finishes.

As your final step, run the sandpaper lightly over every right-angled edge to remove sharp corners that could be dented easily, feel unfriendly to the hand, and might be too sharp to hold the finish. This is sometimes called breaking the edges or softening the edges.

Dewhiskering

Whenever water comes in contact with wood, the wood fibers swell, causing the wood to feel rough to the touch after it has dried. The swollen fibers are often referred to as raised grain. All stains and finishes that contain water raise the grain of the wood. Raised grain telegraphs through the stain or finish causing the surface to feel rough. It can also reduce the depth and clarity of the finish.

Raised grain will happen no matter how smoothly you sand the wood before you wet it. Since you can’t prevent raised grain, the most effective way to deal with it is to make the fibers swell and then sand them level before applying the stain or finish. Once removed, the raised grain won’t reoccur appreciably. Besides dewhiskering, this step is also called sponging, whiskering, and raising the grain.

After sanding the wood to about 150- or 180-grit, wet it with a sponge or cloth to the same extent as you will with the stain or finish. Just short of puddling is about right.

Let the wood dry overnight. Then sand off the raised grain with a sandpaper grit that smoothes the surface efficiently without sanding deeper than necessary. I try to use the same or one-number grit higher than the last grit I used. Dull (used) sandpaper is best because it is less likely to remove more than just the raised grain. If you don’t have any used sandpaper lying around, you can make some quickly by rubbing two pieces of sandpaper together.

Sand lightly. You want to sand just enough to make the wood feel smooth again. If you sand any deeper, you will get below the wood fibers that have been swollen, and you’ll raise the grain again when you wet the wood. (In practice, you’ll always have some additional grain raising, but it will be significantly less.) Always sand in the direction of the grain.

Cleaning Off the Dust

Whenever the last step involves using sandpaper, you will leave dust on the wood. This dust must be cleaned off before applying a finish. There are four ways to remove the dust:

■ Brush it off.

■ Wipe it off with a tack cloth (a cloth made sticky by the application of a very thin varnishlike material that leaves a gummy residue).

■ Vacuum it off.

■ Blow it off with compressed air.

Brushing is usually the easiest and most convenient, but it kicks dust up in the air. If you aren’t working in an efficient spray booth, you should wait until this dust settles before applying a finish.

A tack cloth is most efficient after brushing to remove the remaining dust on the surface. Your bare hand works pretty well too, as long as very little dust remains. (You shouldn’t use a tack cloth with water-based stains or finishes. The varnishlike residue will hinder good flow-out and bonding. Use one of the other methods instead.)

A vacuum is the best way to remove dust if kicking dust into the air might create a problem in your finishing room. Otherwise, compressed air is the most efficient. Use compressed air outdoors or in a well-ventilated space, where the airborne dust will be evacuated.

Though it may seem logical that a better finish will be achieved if absolutely all of the dust is removed from the pores, I’ve never been able to see any difference.

Glue Splotches

No matter how hard you try to avoid it, it’s likely that now and then you will get glue on the surface of the wood during glue-up. Either the glue will squeeze out of the joints as you clamp pieces together, or you will transfer glue to the wood with your fingers. The glue seals the wood, preventing proper stain or finish penetration. You have to remove all the glue from the wood surface, or it will show up as unevenness in color.

Here are some tips to help you avoid getting glue on the wood:

■ Don’t put excessive amounts of glue in the joints. Only when gluing up boards edge-to-edge should you apply glue liberally. In this case you will want squeeze-out to indicate not only that you’ve applied enough glue but that you’ve tightened the clamps adequately.

■ Cut your mortises or dowel holes a little deeper to allow excess glue to collect at the bottom instead of being squeezed out. Also, chamfer the ends of the tenons and dowels and the mouths of the mortises and dowel holes (Figure 2-5).

■ Have both a damp and a dry cloth nearby so you can remove any glue you might get on your hands as you work. Wipe your hands with the damp cloth, then quickly dry them so you won’t wet the wood.

Even following these suggestions, you will still have glue seepage now and then. Here are two good ways to highlight it so you can remove it:

■ Wet the entire surface with any liquid: usually water or mineral spirits. The liquid will soak deeper where there is no glue, leaving the areas that are sealed with glue appearing lighter (Photo 2-4). Water will raise the grain, so you will need to sand it smooth again.

■ Add dye or an ultraviolet (UV) colorant (available from woodworking suppliers) to the glue before you glue up. Dye will show up clearly but will have to be thoroughly removed from the wood. The UV colorant will glow when exposed to UV light.

Figure 2-5: Allowing for Excess Glue

Photo 2-4: Dried glue on the surface of the wood is often hard to see. You can highlight it by wetting the surface, usually with water or mineral spirits. (The upper part of this board hasn’t been wetted.) The raw wood will absorb more liquid, leaving the glue-sealed area appearing lighter.

Removing Dried Glue from the Wood

Once glue on the surface of the wood has dried, there are only two ways to remove it: Scrape and/or sand it off, or dissolve or soften the glue enough so it can be scrubbed off.

Photo 2-5: If any glue remains on the surface of the wood, it will prevent stain and finish penetration and will show up as a lighter splotch.

TIP

If you’re using a water-based pigment stain over glue splotches caused by white or yellow glue, let the stain dissolve the glue. The solvent in these stains (glycol ether), together with the water, will break down the glue so it can be rubbed off with a cloth or scrubbed off with a brush. Simply leave the stain on top of the glue splotch for a minute or so, and then begin rubbing or scrubbing. Keep the area wet with stain. You should see the stain begin to “take” in less than a minute. (This trick won’t work with any other type of stain.)

Mechanical Removal. Scraping and sanding are pretty straightforward in open areas. Use a chisel to get into tight areas around joints, scraping rather than carving away the wood. You have to remove the glue-contaminated surface, then resand the clean wood to the same grit you used elsewhere. This will ensure that the stain or finish colors the wood evenly.

Solvent Removal. You can soften white and yellow glues enough so that they can be scrubbed off by washing with water. Water works better if it is hot, and still better if you add vinegar to it. (Acids soften white and yellow glues, and vinegar is a mild acid.)

A number of commonly available organic solvents can also be used to soften white and yellow glues. In decreasing order of effectiveness, these include toluene (toluol), xylene (xylol), acetone, and lacquer thinner. These solvents won’t raise the grain of the wood as water does, but they will require more scrubbing.

Whichever liquid you use, you will probably have to scrub a little to get the glue out of the pores. A toothbrush usually works, but sometimes you will need to use a soft brass-bristled brush. After cleaning all the glue out of the pores, sand the wood thoroughly to smooth any roughened grain. You can use a coarser grit of sandpaper if necessary, but be sure to finish with the same-grit sandpaper you used on the rest of the piece so the stain or finish colors evenly.

Other adhesives, such as contact cement, cyanoacrylate (Super) glue, and hot-melt glue, can be softened or dissolved using acetone. But epoxy, polyurethane, and plastic-resin (urea-formaldehyde) adhesives will have to be scraped or sanded off.

Removing Glue Splotches After Staining

Despite your best efforts, you may still have glue splotches after staining or finishing (Photo 2-5). Then what do you do? The solution is exactly the same as if you had caught the problem before applying the stain. You have to remove all the glue, and there are still only two ways to do this: mechanically, or with water or a solvent.

After removing the glue, you need to restain, and you may find that the new application of stain is lighter than the original. This could be because the stain that remained in the wood acted as a lubricant for the sandpaper, causing it to scratch less deeply. So even though you may have resanded to exactly the same grit as you used elsewhere, the stain colors a little less.