I’ve had both collectors and artists ask me about varnishing. In my experience, most plein air painters seem to skip this important step. Perhaps it’s because we sell work right off the easel! No time to let the painting dry…
I think varnishing is important, both for the presentation and preservation of your work. I know I was skeptical initially, until I started regularly varnishing, and experiementing with varnish types. I found that the oft-used “retouch varnishes” created too much glare. My goal is to have a uniform surface, one in which the richness of the darks show through, yet don’t overpower the light pigments.
The best solution I’ve found is Gamblin’s Gamvar varnish, mixed with about 10% cold wax medium. Gamvar is mixed by the artist, so you know it’s fresh. You’re given the solvent and varnish crystals. Gamblin recommends you mix the varnish and keep it for a year, no more. I don’t like a strong reflection on the surface, so I mix about 10% cold wax medum. This mixture gives me the depth of color I want, with no glare!
When I see paintings that aren’t varnished, I almost want to get my kit out and do it for them. Look at the difference in this detail of a painting I’m varnishing. I’ve put two strokes of my Gamvar/Cold Wax Medium mixture. The yellow arrows indicate the border between varnished and unvarnished sections of the painting. Look how much richer the darks are! You should also note that it doesn’t have much impact on lights. Varnish brings out the richness of dark to midtone parts of the canvas.