Painting and Technical Advisor for Winsor & Newton, Paul Robinson, says: "What I would advise is that you even out your sheen before varnishing by 'oiling out'. Oiling out is the application of an oil medium to a painting which has sunk (become dull), or lost its oil to the layer underneath. The most common causes for this are an over-absorbent, cheap ground or the use of too much solvent or water and insufficient or no medium.
"When the colour is dry, oil painting medium should be sparingly rubbed into any sunken areas with a clean cloth. As you have used a water mixable oil, I would recommend Artisan Water Mixable Painting Medium which is a water mixable version of the traditional medium. Wipe off any residue and leave to dry for a day or two. If smaller, dull areas remain, repeat the process until the painting has regained an even sheen.
"In general varnishes should not be used for the purpose of recovering the lustre of a dead painting. A decent varnish would still show the difference in sheen from one area to the next."1
If you oil out with just an oil, rather than oil/solvent mixture, it may take a while to dry. Be sparing with it, not to puddle it on your painting.
Gamblin recommends oiling out using "a liberal coating of 1:1 Galkyd Painting Medium and Gamsol Odorless Mineral Spirits to a dry painting"2 to either the whole painting or just an area, leaving this to be absorbed for about two minutes, and then wiping off the excess.
