: How do I store my open can of varnish so that it doesn't skin over overnight?
: So far, I haven't had any luck with any of the air tight containers I've
: tried.

I use a trick that I used years ago with darkroom chemicals. I keep a bag of marbles around (closed to keep dust off). As I use some varnish I put a few marbles into the can. This takes up space and displaces the air so that there is little or no air in the can. Since the marbles are glass, I haven't had any contamination problems. By the time that I am down to mainly marbles, I pour the little varnish left into a smaller container or just use it up in one final session.

John