Varnishing Paintings. By Don Finkeldei: Some answers about varnish and protecting your paintings.
A painting shouldn’t be varnished until it is absolutely dry. That could take up to several months to a year with oil paintings. If you varnish (especially a Damar type) before the oils stabilize and dry, the varnish will merge with the oil paint making the varnish impossible to remove at a later date without damaging the oil painting.
The purpose of a varnish is to provide a protective layer that can be removed when soot and dirt start to affect the color and brightness (possibly a hundred years from now). All varnishes are made with removal in mind. Usually delicately removed with mineral spirits by an art restoration specialist . A new coat of varnish can be applied over the oil restoring the original luster.
Art Conservators remove and restore varnishes when they can. Sometimes they can’t. J.M.W. Turner, the English artist (1775 – 1851) varnished one of his most known and valuable paintings before the oil was completely dry preventing the art restorers at the National Gallery in London from cleaning the painting. It has dulled quite a bit compared to the paintings that they could remove and re-apply varnishes. Another reason artists varnish paintings is to bring out the painting’s colors and unify the luster. Dark paint seems to dry a little duller than bright paint.
Normally I don’t varnish because I can’t wait a year before putting a painting in a gallery. Sometimes I do varnish (after a painting dries for at least a month). In that case, I use a damar RETOUCH varnish (Grumbacher Spray Damar Retouch Gloss) which is a thinner version of the standard Damar varnish. You can apply this before the painting is completely dry. It allows the paint to continue drying, prevents cracking and gives luster to the painting in the interim while the painting dries. I put a note on the back of the painting stating the painting can be varnished with the regular Damar gloss or matte after a year or so. Two coats, sprayed on thinly and let dry between applications at least one day.