Use of retouching varnish
Retouching varnish (sometimes also called intermediate varnish or pull-out varnish) is used to pick up sunken-in (matt) areas and for temporary protection (provisional final varnish) of an oil painting that is not completely dry.
Retouching varnish during the painting process
During painting, ""sunken-in"" spots can occur: the paint dulls, the intensity of the colour diminishes. This cannot be avoided and is caused by a combination of the colour used, the type and amount of thinner added and the absorption of the surface.
The amount of oil in the paint can vary from colour to colour, how much diluent is added likewise. If paint contains relatively little oil, some of which is absorbed by the surface, the colour may sink in.
Assessing the colour harmony of the painting is difficult, with the result that making the right decisions about how to continue painting is also a problem. Treating the sunken-in areas (once they are thoroughly dry to the touch) very thinly with retouching varnish restores their gloss and colour. If the sunken-in areas are very absorbent, it may be necessary to repeat the operation (after intermediate drying) before gloss and colour are restored.
The varnish dries in a few hours and leaves a porous film that is thus suitable for the adhesion of a subsequent coat of paint.
NOTE: It is very important to apply the retouching varnish very sparingly because the paint, which is not yet completely dry, may dissolve in the solvent of the varnish. Preferably apply with a spray can.
Retouching varnish as a provisional final varnish
Retouching varnish can also be applied as a provisional final varnish to give the painting an even sheen and as protection against dirt. Since the varnish is porous in a thin layer, the drying process of the paint can continue.
CAUTION: For this application, it is highly recommended to let the painting dry for at least several months. Even if the paint is hand-dry on the surface at an earlier stage, the paint underneath is certainly not yet. The solvent of the varnish penetrates the underlay and brings out the oil that is not yet dry. If this happens, the painting can remain peckish for months and contaminants such as dust will adhere to the paint.
After the paint has completely dried, a final varnish can be applied over the retouching varnish.