1. Those soluble in water, sometimes called the acid stains.
2. Those soluble in spirits.
3. Those soluble in chemicals.
4. Those soluble in oils.
Two other classes of so-called stains are known as varnish stains and wax stains. These stains are not transparent as they obscure the grain and leave a layer of pigment on the surface.
These four classes of stains may be subdivided into two classes, acid and alkaline, depending upon their chemical reaction with other substances. Water-soluble stains, most largely used, are often made of coal tar dyes, which dissolve in water, and can be used in an acid bath. They are obtained from color substances having no body, such as walnut juice, logwood extract, turmeric, the juice of berries, and the bark of trees.
Stains are applied by brushing, wiping, spraying, and dipping, the latter on quantity production of cheaper grades. Because hardwoods absorb stains more slowly than softwoods, the advantages of the first three methods are apparent. Where this strong contrast between sapwood and heartwood exists, the salesperson should know the sapwood requires more stain than the remainder of the wood. A coat of stain may be applied to the light streaks and after it dries, the entire surface may be stained.
Aside from color there are "polished" and "dull" finishes. Varnish is the original finishing medium, serving as a protective agent and as a means of building up a high finish. For wood finishing the varnish is transparent, but for other uses is sometimes colored, as in black varnish or japan, or by the addition of dyestuffs, as in lacquers.
Lacquers permitting a polish finish are replacing gum varnish finishes to a great extent because lacquer dries in about one-tenth the time required for varnishes, and because lacquer finishes wear well under exposure or use. Chemical action ceases in lacquer films after they harden.