Varnishes. The word varnish has become something of a catchall term applied to a variety of liquid preparations that, when applied to a surface, dry to a clear, hard, and often shiny surface. They are good general-purpose clear finishes.
Natural varnishes are made from natural resins suspended in an oil base (typically, boiled linseed or tung oil); the solvent used is turpentine or mineral spirits. A wide variety of natural varnishes are sold for different uses, from marine applications to gymnasium floors to furniture. Exterior varnishes tend to have more oil, those for interior use less. So-called long oil has the richest mix of oil, so its strength is waterproofing, making it perfect for the boatyard; medium oil is very durable, so it’s the right candidate for floor finishes. Short oil leaves a hard and brittle finish, suitable for furniture.
Polyurethane is a synthetic varnish. It is durable and dries quickly to a clear and transparent finish. It is sold in a variety of different formulations, both oil- and water-based. Polyurethanes and most other varnishes can be bought in glossy, flat, or satin finishes.
Varnishes are more difficult to use than, say, shellac or penetrating oils. They dry slowly (which means that dust and debris can accumulate on their tacky surfaces as they dry) and the surface tends to bubble. When applying varnish, the room should be warm and as dust-free as possible. Stir the varnish before using it, but don’t shake the container, as that will cause bubbles to form.The brushes, too, must be clean. Overlap your brushstrokes, and apply the varnish in a very thin layer. Don’t over-brush. After varnishing an entire surface, go over it once carefully with the tip of the brush. Don’t apply more varnish, but smooth the varnish already there in long, even strokes.
When the varnish has dried completely, smooth it using extra fine sandpaper or steel wool. Apply a second coat. After that has dried, you may wish to rub the finish with steel wool or pumice. Apply a paste wax, and buff the finish thoroughly.
Paints. Varnish, oil, shellac, and French finishes are, for the most part, clear finishes. Paint, in contrast, is opaque, and usually contains colored pigments.
The pigments are finely ground solids that are suspended in the base material, which may be latex, oil, or other substances (lead-based paint now is no longer available because of the toxicity of the lead). Paints are sold for use indoors and outdoors (exterior paints are much more durable, resisting fading, chipping, and peeling). Most are sold in a variety of lusters, depending upon whether they dry to a glossy sheen; a semigloss or satin finish; or a flat or eggshell surface. Gloss and semigloss have the virtue that they can be washed and even scrubbed; flat or satin finishes have a soft, understated presence.
Before being painted, raw woods need to be sealed with a primer coat. This seals the wood and prepares a surface to which the paint to follow can bond. The primer used should be matched to the paint to be used; read the instructions on the can or consult your local paint supplier.