Choosing A Finish

Choosing A Finish
Copyright 2000 by Jeff Jewitt
Not reproducible in any form, written or electronic, without permission

Undoubtedly, one of the continuing bugaboos for woodworkers is finishing. Though undaunted by complex joinery or intricate and precise machining, many woodworkers still cringe at the thought of applying a finish to their work. “What’s the best finish for my project” is one of the first hurdles to overcome.

Of the hundreds of products sold as finishes to woodworkers, all finishes can be grouped down into more manageable categories having general working qualities and degrees of protection. These groups are waxes, oils, varnishes, shellac, lacquer and water-based finishes. Different finishes have varying degrees of protection, durability, ease of application, reparability and aesthetics. Unfortunately there is no finish that excels in all of these categories. So choosing a finish is about trade-offs – a finish that excels in one category may fail in another.

As a professional refinisher, I routinely ask my customers a series of questions so I can determine the best finish for their furniture. They’re designed to give me an accurate picture of what finish is appropriate, and I accompany these with finished samples. I’ve modified these questions a bit and added a few so that woodworkers can determine what finish to use. When you choose a finish you should consider the following.

1. How will the item be used? Is it subjected to a lot of moisture, solvents, food, scrapes and dents? Or will it be primarily a display piece.

2. What is your skill level and work area – does it stay clean and is it heated and dry?

3. What do you want the wood to look like – do you want an “in the wood” natural look or a thicker finish that accentuates depth?

4. Will you be filling the pores?

5. What color do you want the finish to impart to the wood – do you want the finish to alter the color of the wood. Is yellowing an issue? Do you want to minimize the wood changing color as it ages?

6. Will you rub the finish out to the sheen you like?

7. Safety and health – are you sensitive to certain solvents, is flammability a problem, or are you concerned about the environmental impact of certain finishes

8. Toxicity of the finish – will it be used in food preparation?

Keep in mind that finishes are generally divided into two distinctly different “types”. Evaporative finishes are those finishes that dry or cure to a hard finish by evaporation of the solvents in the finish. These finishes are lacquer, shellac, and water-base finishes. Reactive finishes are finishes that cure by reacting with another component – either outside the can (like air) or placed in the can right before application. Examples of reactive finishes are oils, oil based finishes, and catalyzed lacquers and varnishes. General working qualities are similar within these groups like reparability and dry time (the exception being pure oils like tung and linseed oil)

Use (durability)

This category comprises the durability of the finish and its resistance to water, chemicals and solvents (like alkaline cleaners and acidic foods), heat and scratches. As a rule, evaporative finishes are less durable overall than reactive finishes (with the exception of true oils). Waxes, shellac, lacquer and some water-base finishes can be damaged if exposed to water long enough. Shellac is not resistant to alkalis like ammonia. Most of these products will scratch easily, however, they rub out well (that’s the flip side of scratch resistance). Waxes are surprisingly resistant to acids and alkali’s, however, aside from this they are the least durable. Of all the evaporative finishes, lacquers like nitrocellulose and the acrylics (both water-based and solvent ) fare the best for overall durability.

Reactive finishes (other than pure oils) are much more durable than the above finishes. Oil-based polyurethane is the most durable finish that you can apply by hand, while catalyzed lacquers and varnishes are the best for spray application.

Skill Level

Obviously, your degree of skill, the environment you have to work and the tools you have access to play a part in choosing the best finish. If the environment in which you have to work is cold, or there’s constant sanding dust in the air from running machines, the evaporative finishes are the easiest to work with. Because these finishes dry fast, dust falling in the finish does not pose as great a problem as slow drying finishes like varnish. Shellac and lacquer are the least temperamental when it comes to cold temperatures, and can be modified with additives (retarders) for hot and humid conditions. Oils and oil based products dry slowly in cold temperatures and humid conditions, and dust is always a problem as they remain tacky for a long time and dust becomes embedded in the dried film.

Spray equipment requires a larger budget, and in some cases requires equipment to exhaust the overspray. There’s also a learning curve in spraying, so it may take a bit of practice before decent results can be expected. However, the speed and subtle control over applying finishing products is well worth it for most, even with modestly priced equipment.

Aesthetics

This category has to do with how you want the wood to look. Do you want a natural, “close-to the-wood” finish. Or do you want an elegant, deep, glass-smooth finish that accentuates depth and luster. Is the color of the finish a problem, or will yellowing of the finish be a problem later?

Traditionally, woodworkers have turned to the oils (tung and linseed), wax, or oil-varnish blends (like Watco) for a natural finish. These easy-to-apply finishes do not form a hard film when dry, so they can only be built up to a certain point on the surface of the wood. However, any finish can be used for this effect. As long as you don’t build up the finish beyond several coats, shellac, lacquer, varnishes and catalyzed finishes can all be used for natural-looking finishes.

Hard, film-forming finishes like shellac, lacquer or varnish have to be used if you’re going for a filled-pore, deep, lustrous finish. These finishes also are used when complex coloring options like toning and glazing are used.

The color and the penetration of the finish itself may be an issue. Orange shellac or tung oil/phenolic resin varnish both may have a color that’s too dark for woods you may want to keep as light as possible. In addition many finishes deepen or darken the wood surface. In most cases this is desirable, because it kicks out depth and increases luster. However, you may want to downplay the deepening effect. Some delicate figured woods may turn to a muddy appearance with an oil finish. As a rule, oils and oil based varnishes deepen color of the wood and increase luster the most, followed by solvent based lacquers and shellac. These finishes “wet” the cells of the wood – penetrating into the surface. Other finishes tend to lay on the surface and do not penetrate which has the effect of keeping the wood lighter. These finishes are most water based finishes based upon polyurethane or acrylic, waxes, and some catalyzed finishes.

The “plastic” look that’s sometimes ascribed to finishes like polyurethane and catalyzed lacquers has to do more with incorrect application of these finishes than the finish themselves. On open-pored woods, applications of thick, high solids varnishes and lacquers results in a “soupy” look to the pores, a result of the finish bridging the pore rather than flowing into it. Proper application of these finishes by thinning them results in a more attractive look. My favorite method for oil-based polyurethane application is to thin the finish 50% and wipe it on.

Yellowing of the finish is a problem over white and painted finishes (yellowed lacquer over a blue paint will turn it green) or if your intentions are to retain the natural color of the wood as much as possible – as in maple or birch. Acrylic based finishes, both water and solvent based are the best choice for non-yellowing finishes. Waxes also will not yellow as well as some catalyzed lacquers and varnishes.

Safety/environmental

Solvent based finishes like varnish and lacquer contain a good deal of organic solvents, which create an environmental and health impact and they are flammable. If this is a problem for you, use a water-based finish to eliminate the fire problem and mitigate the environmental and health impact. Oils are a surprisingly good alternative to solvent based lacquers and varnish, they are 100% solids (no solvents), and come from renewable resources however, oil-soaked rags must be disposed of carefully. Shellac is also a good alternative. The solvent for shellac, ethyl alcohol, is distilled from corn and for most people, the fleeting odor isn’t objectionable.

All finishes are non-toxic when fully cured, despite what you may have read or heard. Once the solvents are evaporated, the cured film is safe enough for contact with food. This does not mean that the finish itself is safe to gobble up. It means that additives such as driers or plasticizers are encapsulated enough so that they do not migrate to what you’re eating. For edible finishes, wax and shellac are the only ones I’m aware of (which is why apples and candy are coated with these)

Spraying wastes a good deal of finish and the organic solvents are dispersed into the air. Brushing or wiping on a finish is a practical, though less speedy alternative

The two drying oils used in finishing are linseed oil and tung oil. Other drying oils have been used, but these two are easily available and inexpensive. In an effort to distinguish these two products from other finishing products hyped as oil finishes, we call these true oils. The conversion of the liquid oil to a solid is through a process called polymerization.

Linseed oil is available in several forms. Linseed oil in unrefined form is called raw linseed oil and is rarely used on wood because it dries very slowly. Our ancestors realized this but found that if the oil was heated (boiled), the resulting product was thicker and dried quicker. Though real heated oil is available (called heat-treated or polymerized oil) boiled linseed oil made today uses a slightly different process – the raw oil has chemical additives which speed up the drying time and oxygen is bubbled through the oil. For wood finishing purposes boiled linseed oil should always be used.

Tung oil is derived from the nuts of certain trees indigenous to Asia and other parts of the world. Tung oil is available in pure, unrefined form or heat-treated in a form called polymerized tung oil. The heat-treating process makes the oil a tad bit more durable and speeds up the dry rate. It also minimizes the tendency of tung oil to “frost” or dry to a whitish, matte appearance. It is a bit paler in color than linseed oil and has better moisture resistance than linseed oil.

Both linseed and tung oils are penetrating finishes – which means that they penetrate into the fibers of the wood and harden within the wood. These are the easiest finishes to apply – they are wiped on, allowed to penetrate and the excess wiped off with a rag. They are generally not built up to a surface film like varnish or lacquer because the film dries too soft.

Varnish refers to finishes that are made from drying oils like linseed or tung and tough, durable synthetic resins. Some varnishes are simply solutions of the resin and oil, like phenolic resin varnish. Others like alkyd varnishes and polyurethane are really drying oils like linseed, tung or soya oil that's modified with chemicals called polyols and acids. This forms an oil/resin structure called an alkyd. Because alkyds contain residual oil they dry by the same process as true oils – polymerization, but the resin makes the finishes more durable than oils. In fact, oil-based varnishes are the most durable finish that can be easily applied by the average finisher, surpassing most of the other finishes in water-resistance and resistance to heat, solvents and chemicals. There are three general groupings of varnish. Those based upon a high percentage of oil in the finish are called long oil varnishes. Medium length varnishes have a lower percentage of oil and short oil varnishes even lower. The ratio of oil to resin affects the hardness of the varnish. Long oil varnishes are also known as marine varnishes, spar varnishes or just plain exterior varnishes and are elastic and softer than short oil varnishes. Short oil varnishes are for industrial use and require baking to cure properly. Medium and long oil length form the basis of most wood finishing varnishes.