Retouch varnish – intermediate varnish to restore gloss to dry oil paint for use when you want to continue working on or reworking areas of a dry painting. It contains much more turpentine than a final varnish, so it doesn’t leave the painting with a permanent gloss.
When a painting dries, it often does so unevenly, and certain areas may appear dull or flat while others may look glossy. Retouch varnish will impart a uniform gloss to the entire painting, making it appear as though it is wet.
If you try to work on a dry painting without it, you may have trouble matching colors, because the color you apply will usually dry darker than the rest of the painting. e.g. use spray damar retouch varnish, which allows you to apply a uniform, thin layer easily. Use it sparingly and with adequate ventilation.
Damar (a natural substance) and several synthetic varnishes come in undiluted form for use as a final varnish, which is applied to protect the painting from dirt. Instead of lodging on the paint surface, dust lands on the varnish and can be wiped off easily. Final varnish comes in a matt, semigloss, or a high-gloss finish.
A final varnish can always be removed with the proper solvent, often a mixture of turpentine and a small amount of alcohol.