The photo below is a detail from one of Theresa's portrait paintings. It is a 32 x 48 oil painting of a woman and four horses. She finished it this past summer and we have been displaying it at shows ever since. A couple of weeks ago, between shows, I got the chance to varnish the painting. This is not polyurethane or a varnish you get at the hardware store. Picture varnish is an entirely different animal. First, it is clear. Second, while it protects the painting, it is delicate compared to modern wood varnishes. Third, it is reversible. It can be removed at a later date without damaging the painting.
What it does do it take the painting to another level. The darks become richer and the lights pop out. Think of the difference between a car with a beautiful shiny paint job and one painted with flat grey primer.
Look at the difference between the horse's head on the right and the one on the left. The fence board really shows the difference between the dull unvarnished and the varnished side. Note the girl hair, the side on her left has been varnished the right side not.