Oil varnish, which consists of resin dissolved in solvent (or a mixture of solvent and drying oil), is used when the painting is in progress or after the work is completed.
When the painting is in progress, a mixing varnish is added to the painting medium to accelerate drying time and to add gloss.
An isolating varnish (which is insoluble in solvents) 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 as temporary protection against dust and moisture or to create 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 finishing varnish is applied to the surface as a final coat to protect paint film from atmospheric deterioration and contact abrasion. Picture varnish is also used to unify the appearance of the surface to the desired gloss, satin, or matte finish.