The necessary conditions for all good varnishing are a perfectly smooth, even, filled surface of dry wood, a temperature of about 70 deg.

and no dust in the air.
In general, there are two classes of varnish, based on the character of the solvent, (1) Spirit varnishes and (2) Oil varnishes.
(1) Spirit varnishes are sometimes made with copal resins dissolved in some spirit, as one of the alcohols, benzine, acetone, etc.

They dry with great rapidity owing to the volatilization of the solvent spirit, leaving a coat of pure resin of great hardness and brilliance, but one which is likely to crack and scale when exposed.

They are not much used.

Shellac is the most common and the most useful of the spirit varnishes.