Oil varnish consists of resin dissolved in solvents or a mixture of solvents and drying oils. The three types of oil varnishes are as follows: mixing, isolating, and final. These varnishes have different functions and are applied at different stages of work.

When the painting is in progress, a mixing varnish is added to the painting medium to accelerate drying time according to the needs of the artist and sometimes to add gloss. An isolating varnish (which is insoluble in solvents) can be used during painting. An isolating varnish is applied to the paint film of bottom layers to prevent them from being affected by solvents in the next layer of paint. Retouch varnish is applied in between painting as temporary protection against dust and moisture or to assist the artist in the painting process by creating a uniform wet look that remedies the unevenness of light being reflected from wet and dry parts of the painting.

After the work is completed, a picture or final varnish is applied to the surface to protect the paint film from atmospheric deterioration and contact abrasion. A final varnish is also used to unify the appearance of the surface to the desired gloss, satin, or matte finish.