One thing you can count on when you save a half-used can of varnish is that the contents will be skinned-over the next time you open the can. When this happens, don't gnash your teeth; the varnish underneath the skin should be just fine. Before
you use it though, be sure to do two things: First, strain the varnish
through one of those paper funnels with a mesh bottom (available at
paint stores), some doubled-up cheesecloth or old pantyhose. This will
get rid of any small bits of dried varnish that are floating around.
Second, try the varnish on a piece of scrap to make sure it dries OK.
If you have any doubt, toss the old stuff out and buy new. It's just
not worth the risk.