Varnishes may be divided into the following four categories, depending upon the type of solvent used:
Oil varnishes
These varnishes use linseed oil as solvent in which hard resins such as amber and copal are dissolved by heating. These varnishes dry slowly, but form hard and durable surface. Sometimes, small quantity of turpentine is added to make the varnish more workable. Oil varnishes are recommended for all external wood work, and for joinery and fittings.
Spirit varnishes or lacquers
These varnishes are methylated spirit of wine as solvent in which soft resins such as lac or shellac are dissolved. They dry quickly, but are not durable. French polish is a variety of this type varnish. It is commonly used on furniture.
Turpentine varnishes
These varnishes use turpentine as solvent in which soft resins such as gum dammer, mastic and rosin are dissolved. The varnish dries quickly, but is not so durable. These are cheaper than oil varnishes.
These varnishes are formed by dissolving shellac in hot water, using enough quantity of ammonia, borax, potash or soda. Water varnishes are used for varnishing wall papers, maps pictures, book jackets, etc.
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