Like Elizabeth, I use Gamvar. It can be applied as soon as the paint in dry to the touch. I usually wait on the varnish until I see a bid go up on the painting. Once I see that, the varnish goes on, along with the info card that gets glued or attached to the back of the painting. Gamvar dries in about 24 hours, so I have no problem getting a painting out within two days after it’s sold.