The varnish is the protective coat applied to the paintings surface. The choice of the type of varnish as well as the applying method are important for the artwork since those two factors heavily influence the final appearance of the painting. The choice is made according to the need of saturation or the wished gloss level or mattedness. In any case, as a restorer, we always have to think about their compatibilité, their aging steadiness and be sure about their reversibility.
For example, the retouching varnish, applied before the filling and inpainting to saturate the colours and isolate the original paint from any compensations, has to have other features than the final varnish. The latter homogenise the surface appearance and contains uv-protection.
The application can be made with a brush, in a « French » way, with an aerosol or with an air compressor.