This is a good topic for both artist and collector. Artists must decide if, when and how to varnish. And as a collector, you should be aware of how the painting you purchased was or wasn't varnished. I recently spoke with the folks at Gamblin about varnishing (I use their Gamvar varnish) and with their permission, have included a link to their varnishing studio notes here.
I have multiple reasons for varnishing. First, it is an aesthetic
decision. As I'm painting, all values and colors relate to each other
when wet. As they dry, they change color slightly and look less
saturated. Varnishing will unify the gloss of the surface and saturate
color. Since my original intent was the values and color I saw when
wet, I varnish in order to retain that look. Varnishing can also
protect against dirt and in Gamvar's case some UV light, though
protection from UV light is at it's best when freshly mixed.
Gamblin's synthetic resin component system was developed by conservation
scientists at the National Gallery. Unlike natural damar resins,
Gamvar won't yellow or darken. It can be removed with a mild solvent
such as mineral spirits, while damar varnish is harder to remove,
endangering the top layers of paint, especially if glazes were used.
The component system allows for mixing when needed which makes the
varnish the most effective. I mix when I'm ready to varnish a group of
paintings, date the mixture, and use it within one month. I mix Cold
Wax Medium into the varnish to create a more matte varnish, then apply
two thin coats to each painting, waiting 24 hours to dry in between. I've
started to write the date I varnished the painting on the back of each
painting so the collector who buys the piece will know when and what
Bottom line, as a collector, you should know
what was used on the painting you purchased, if anything. Artists
should research this for themselves and decide what's best for their