A good wood finish typically protects wood while allowing the natural beauty of the grain to shine through, which differentiates it from paint or other opaque coatings. You'll find a variety of finishes at most hardware stores, and although you might think there's a single best one for each application, that isn't necessarily true. A number of considerations enter into your decision. Do you want the finish to be matte or glossy? Do you prefer natural wood grain texture to a smooth film coating? Does the wood need protection from sunlight, impact or foot traffic? How much time are you willing to devote to finishing? A basic understanding of the characteristics of different finishes will help you make up your mind.
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Film Finish or Drying Oil
Penetrating oils protect from within, while film finishes remain on the surface and provide impact resistance. The two most common penetrating oils are tung oil and boiled linseed oil. Both soak into the grain and polymerize, forming a water-resistant barrier that allows the natural grain to shine through. Film finishes include: varnish, which may or may not contain polyurethane; lacquer; and shellac. These finishes are the best choice if you want the wood to assume a smooth, glossy appearance.
Tung and Linseed Oil
The classic penetrating oils both come from natural sources. Tung oil is derived from the seeds of the tung tree, which grows in Asia; and linseed oil, which is produced by pressing flax seeds. Pure linseed oil won't harden—it has to be boiled or double boiled to make it suitable for wood finishing.
The use of varnish dates to the ancient Egyptians, and there are a variety of formulations. Traditional varnish is a mixture of solvents and plant alkyds, while more modern types include plastics such as acrylic and polyurethane. Varnishes are typically slow drying, and they harden by polymerization, or cross-linking, as they cure.
Wood Finishing Oils
Commercial finishing oils are usually a mixture of varnish and penetrating oil. The exact proportion of each in any particular product may not be specified on the container. Finishing oils often also contain wood colorants.
Oriental woodworkers are primarily responsible for the two main evaporative finishes available today. Shellac is a product made by dissolving secretions of the lac bug, which lives in Thailand and southern India, in alcohol. Japanese craftspeople originally made lacquer from urushiol—the sap of the Rhus verniciflua tree, which is related to poison ivy. Modern lacquer is made by dissolving nitrocellulose in a soup of strong solvents that carry the generic name lacquer thinner. Both lacquer and shellac provide clear plastic coatings that harden as soon as the highly volatile solvent evaporates.
Ease of Application
Penetrating oils are the easiest finishes to apply. You typically rub them onto the wood with a rag or paint them with a brush, wait a few minutes, then wipe off the excess. At the opposite end of the scale, evaporative finishes can be finicky to apply because of the volatility of the solvents carrying them. You can apply shellac with a brush, but unless you have a formulation designed for brushing, lacquers must be sprayed.. The much slower drying time of varnish, on the other hand, makes it perfect for application with a paint brush.
Because evaporative finishes dry so quickly, you can layer them on and achieve a really smooth, durable coating in a short time. You can also do this with varnish, but it takes days to do so rather than several hours. When using a penetrating oil, there's little advantage to applying more than one or two coats.
Best Exterior Finish
Virtually all wood finishes are suitable for interior woodwork, but when it comes to outdoor applications, varnish is the only coating that you should consider. Penetrating oils and evaporative finishes generally deteriorate too rapidly to provide any real protection, although you may find a penetrating oil formulated specifically formulated for exterior use.
An exterior finish must protect against ultraviolet sunlight as well as moisture, which is almost a proscription against using a clear finish. However, spar varnish has enough pigmentation to fill the bill. Spar varnish is typically formulated with alkyds, although it may contain some polyurethane for added durability. It's a better choice than pure polyurethane varnish for finishing outdoor furniture, as well as exterior doors and woodwork.
Best Floor Finish
While craftspeople in days gone by finished floors with penetrating oils and shellac, most modern floor finishers use polyurethane varnish. It's durable, easy to apply and levels out to a smooth, attractive finish. Glossy sheens are highly reflective, but the muted tones of semi-gloss or flat floor finish are less distracting and more popular. The additives that control sheen have little to no effect on the durability of the finish. A typical floor finish requires three coats of varnish.
Best Finish for Interior Woodwork
Furniture manufacturers typically have a spray booth in which they apply a lacquer finish to their products, and you can do this at home with a spray can or a can of brushable lacquer. While lacquer is the coating of choice for tabletops, cabinets and other chairs, you might also consider shellac or polyurethane varnish. Both of these coatings tend to turn yellow as they age, though, and are probably more suitable for interior trim and woodwork features than they are for furniture. Both lacquer and polyurethane varnish are available in gloss, semi-gloss and flat sheens.
Water- vs. Solvent-Based
Varnishes and lacquers come in water-based formulations as well as solvent-based ones, and the water based varieties have many advantages. They dry more quickly, they clean up with water and —very important—they contain much lower levels of volatile organic compounds (VOCs).
Choose a water-based product if you're in a hurry or sensitive to paint odors. Keep in mind that, although the durability of the finish is unaffected by the solvent, the finish of a water-borne product may appear more plastic-like. This is because the faster evaporation of water doesn't give the finish as much time to level out, and the final film isn't as flexible. Water-based finishes tend to form bubbles in the container, and these bubbles can dry into the finish. To avoid this, stir the finish slowly to avoid creating turbulence.