Whether or not you are planning to refinish an old piece of furniture or simply finsih a newly purchased unpainted piece, your first step is to decide what finish you want to apply -- and this will depend largely on the effect desired.
Some finishes hide or mask most -- if not all -- the original color and grain pattern. Under these you can use patching compounds or make repairs without worrying too much about matching color and grain. On the other hand, clear or transparent finishes let almost everything show through, so preparatory patching must be done carefully to achieve a uniform job.
As a rule, all finishes applied to wood furniture fall into two categories: opaque, or heavily pigmented semiopaque finishes that mask or hide the wood's original color and grain pattern, and so-called "natural" or transparent finishes that allow the natural color and grain to show through (a stain or lightly pigmented sealer may be used to change the color, but the original grain and color of the wood still show through and affect the final tone and appearance).
In deciding between the two finishes, the first consideration is the species of wood and the condition of the piece. If it is unattractive wood with very little grain, if it has a great many blemishes that will be hard to hide, or if it has parts that differ in color, you may select an opaque painted finish -- such as a solid color enamel, or one of the various "antique" or glazed finishes that require an opaque coat of low-luster enamel as a background coat.
On the other hand, if the piece has an attractive grain and the wood is in fairly good condition, you will probably want to use a transparent finish such as varnish, lacquer or penetrating sealer -- using the clear finish directly on the wood, or after a lightly pigmented stain has been applied to darken or highlight the grain.
The next consideration is the effect and color wanted. If a wood grain or natural look is your goal, you will want a transparent finish through which the grain shows clearly. If you want a bright or solid color, you may use an opaque enamel finish, or one of the various multitoned antique or glazed finishes.
When refinishing older pieces there is often another factor to consider before deciding on the finish: Are you willing to go to the trouble and expense of completely stripping off the old finish, and perhaps bleaching the wood after you have stripped it bare, or do you simply want to be able to go over the old finish with a minimum of cleaning, sanding and scraping?
Opaque finishes (paint, heavy stain) can be applied without completely stripping off the old finish, but clear or natural finishes require complete removal of old finish down to the bare wood.
When you use paint remover to strip off old finish, it does not remove the stain -- only the surface coating (varnish, shellac, etc.). Therefore the color of the wood will remain basically the same although it will be a bit lighter. This means that if you want the wood much lighter, or a completely different shade, you will have to bleach it, then apply a new stain in the color you want. You can always stain wood darker, but seldom can you effectively stain it lighter.
In addition to using an opaque paint to completely hide the original wood grain and color, there are also some heavily pigmented oil stains and latexbase stains that can be used to hide don't feel stripping is worth the effort. These are somewhere between paint and stain in hiding power, but they still give a wood grain effect when applied after the old finish has been thoroughly sanded and cleaned. You can then apply varnish or other clear finish over them in the usual way.
The clear or natural finishes are much more popular because they permit the wood grain to show through and are richer looking. These can be applied directly to the wood when the basic color or tone is to remain as is, or they can be applied after stain has been applied to change the color to the desired tone.
Remember when choosing a stain that all clear finishes will darken the final color to some extent, and that stains come out differently on different kinds of wood. To be sure of the final result, experiment on a scrap piece of the same kind of wood.
Clear coating fall into two categories: surface coatings and penetrating finishes. A surface coating is one that lies on top of the wood and builds up as more coats are applied. Included in this group are varnish, shellac, lacquer, clear vinyl and urethane (actually the last named is simply another form of varnish). Since these all remain on the surface, they may crack and peel, and will show scratch marks to varying degrees. They can be used to achieve anything from a very high gloss to an almost dull finish.
Penetrating finishes differ in that they soak into the wood, filling the pores and leaving little or no surface film when properly applied. As a result, there is no surface coating that can get scratched (unless you go deep enough). This type is often referred to as Danish oiled finish, and it works best on hard woods or those with an attractive grain and texture -- teak, walnut and oak, for example.
Penetrating finishes -- also referred to as sealers -- can only be applied to raw wood, or to wood that has the same kind of finish already on it. They give a full oiled or waxed appearance that is particularly suitable for hard woods with attractive grain and texture, and they are easies of all finishes to maintain. They can be touched up easily and generally do not show scratches, but they still need protection with wax and may not be appropriate for some traditional furniture that normally has a built-up glossy surface coating.