Does wood actually need to be finished?
For some situations of indoor and outdoor use, the answer is no.
Wood left outdoors to weather naturally is often attractive just by itself; cedar, cypress, osage orange and redwood are naturally resistant to rot and decay.
Some indoor furniture and architectural styles also favor the use of hand-planed pine and fir which has no protective finish at all.
In most situations, however, some sort of coating on the wood is beneficial in these ways:
- It prevents the accumulation of dirt, which is difficult to remove from raw wood.
- It slows down the exchange of moisture between the wood and the air, reducing shrinking and swelling of the piece.
- It deepens and clarifies the appearance of the wood's cut or sanded surface.
- It helps retain that clarity and the wood's original color over time.
Another reason to apply a finish is to change the original color of the wood, to accentuate, deepen or clarify its appearance. Staining or coloring the wood often improves its look, particularly in light-colored or bland woods, but it is not a necessary step in finishing. Staining and finishing are actually two separate processes, and the wood can be finished with or without staining. Many woods look handsome enough on their own, with just a clear protective coating.
Here are some common finishes and their characteristics:
- Wax Finish
A finish consisting of nothing but paste wax is often enough, especially if the wood will not be exposed to alcohol or water. Wax has the interesting property of being able to stick to anything but itself. When a wax finish ages or fades, it can be renewed simply by applying more wax, which will both remove and coat over the old wax. Some woods are especially well suited to wax finishes, such as Hard Maple, Beech and most dense tropical hardwoods.
In addition, wax can be used as a protective coating on top of almost any other finish, where it will prevent abrasion, partly because of the slipperiness of the wax coating.
- Oil Finish
Oil finishes are usually a type of linseed oil, though often some varnish may be added for durability and additional protection from moisture and solvents. Three to five coats of oil will produce a pleasing sheen.
Like wax, oil finishes are susceptible to stains from alcohol or water, but make a good natural protection against dirt and grime. They also benefit from a renewal of the finish periodically, by cleaning and then reapplying the same material.
Some mention should be made of shellac, a relative of lacquer. For thousands of years it was perhaps the finish of choice for furniture and woodwork, and is also the primary finish material in French polishing, a difficult and labor-intensive process that produces one of the best-looking finishes there are. Nowadays, so many other, tougher finishes are available that it is rare to see shellac used as the only finish on a piece of furniture.
Part of the problem is that any piece finished with shellac is vulnerable to alcohol damage. Since alcohol is shellac's primary thinner, it can easily be damaged by a drink spilled on the surface. It is also somewhat vulnerable to water damage.
Shellac does find a use as a sealer or barrier coating underneath other finishes, or between layers of finish. It can prevent stains or oils from migrating up to the surface and ruining the top coating, or can seal in stain colors so they don't bleed into subsequent layers.
Varnish finishes are made of resins which, when cured, form a film which is chemically different than it was before. That means that the final finish will be unaffected by the solvents which could thin or destroy the uncured varnish. Varnish--especially urethane or polyurethane varnish--is a tough, durable finish, and relatively easy to apply. It is available with a variety of sheens from glossy to flat.
Varnish's ease of application makes it ideal for the do-it-yourself project, since all that's needed is a brush and a well-ventilated space. Varnish can even be wiped on, then smoothed off, like an oil finish: this makes a very smooth finish, though it may take four or five coats to get the thickness needed.
- Lacquer Finishes
Lacquer is made by an insect, called the lac bug. The lacquer is thinned in a solvent, then applied to the wood; the solvent evaporates and leaves a coating of lacquer. It gives an excellent protection against water, dirt and many chemicals. Because it can always be re-dissolved in thinner, the cured finish can still be damaged by alcohol or solvents, but is well suited to the majority of furniture and woodwork.
One of lacquer's problems lies in its difficulty of application: most lacquers can only be applied with spray equipment.
Yet more difficult to apply,
catalyzed lacquers are composed to cure chemically, so the resulting finishes can be highly resistant to all sorts of caustic chemicals. They make excellent coatings for kitchen and bathroom fixtures and cabinets.