If you put the top on very securely, and store the can upside down, the skin will form on the top of the liquid, which will be the bottom of the container the next time you open it. The rest of the varnish will be in fine shape. Store the can in an old bowl, just in case it leaks.

A couple other tips. Pour off only a little more than you expect to use, into a separate container. When done, let it skin over and toss it - don't pour it bqack in with the new stuff. Also, don't thin the new stuff in the original can. Some professional yacht refinishers thin each coat a different amount, e.g. 10% on the first coat, 20% on the second, etc., but only in the "working" container, not the bulk can.