Let's start by dividing finishes into types. The most common wood finishes are oil, varnish, shellac, lacquer, water-based and wax. Sometimes, these finishes are mixed and given new names. "Danish oil," for example, is usually a mixture of oil and varnish. A "tung oil" finish is usually varnish thinned with mineral spirits. When finishes are mixed, they take on some of the characteristics of each component.
As you get to know these products better, remember this simple maxim: the purpose of finishing is to enhance the wood's natural beauty, seal it, and protect it. Enhance, seal, protect.
Each type of finish on those shelves performs differently. In making your choice, ask these four questions:
• How will the finish affect the wood's color?
• How well does the finish protect the wood?
Color. Each finish affects a wood's color in a different way (see 6 Finishes, 6 Colors, right). Sometimes the effect is desirable, sometimes not.
Application. To a large extent, the method of application is a matter of personal choice, but some methods require more skill and practice than others. An oil-type finish or a wax is the easiest to apply: you just wipe it on and wipe it off, leaving a thin film. Shellac, varnish and water-based finishes are usually applied in a thicker film with a brush, which often requires deft handling to avoid runs, dry spots and lap marks. Lacquer is almost always sprayed. Unless you use an aerosol can, spraying requires extra equipment, such as an HVLP sprayer or compressor, and practice beforehand.
Protection. If you want your project to have a long life, choose a finish that offers maximum resistance to the exchange of water vapor between the wood and the air around it. The thicker the finish coat, the better it will slow this exchange. That's true for each type of finish. There is a limit to the effectiveness of thick coats, though. Any finish greater than about 0.006 in. (roughly four coats of polyurethane) has a tendency to crack, especially with sudden changes in the weather.
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