When you work on an oil painting over an extended period, colors sometimes become dull. The upper paint layers merge with an earlier layers. This process is called sinking in. If the problem isn’t corrected, it gets worse as the painting progresses.
A final varnish can mitigate the bad effects of sinking in, but how do you fix the problem on an unfinished painting? Retouch varnish is designed for this situation. Retouch varnish is a light, fast-drying varnish that protects new layers from sinking into the old ones. When a painting is in-hand for a long time, retouch varnish can be used repeatedly. Retouch varnish serves the same purpose for oils as does spray fixative for pastels.
Here is a painting on which I applied a single layer of retouch varnish. Some of the grays sank in, and the coat of varnish enlivened them and corrected the problem. I was able to finish the painting, Woman in Red Pants, in a single session after applying the varnish.
So, what is retouch varnish? Generally, retouch varnish is a resin-turpentine varnish. Artists being artists, a lot of other concoctions are seen. Norman Rockwell used shellac. Because shellac is dissolved in alcohol, it’s super-fast drying.
For Woman in Red Pants, I used Grove’s fast-drying copal varnish thinned 8 to 1.
This photo shows some of the varnishes I’ve purchased. I bought the copal-turp retouch varnish from Doak some time ago. I don’t think he makes this particular product any longer.
Rublev’s mastic varnish is decent varnish, although I don’t use it. I prefer the harder copal resin varnish. Rublev doesn’t make this varnish anymore either.