Take a plastic bag and fit it down into the can with the opening of the bag around the top. When you put the bag down on the top of the varnish, you take the air away from it. That is what is causing the varnish to start setting up. If you want to put something in the bag to help fill all the area around the top of the varnis, use a little of the thinner you use with the varnish. That way if the bag leaks some, you don't ruin your whole can of varnish. You just have to stir it up. I was having the same problem and this solved it.

: I've had a quarter can of Z-Spar Captain's and a quarter can of Z-Spar
: Flagship varnish turn to jello right in the can almost overnignt. Once it
: starts it seems impossible to stop by thinning. I always keep the lid on
: except when pouring from the can. I thin 10 to 20 percent in the can with
: Z-Spar T-10 to apply here in sunny So. Cal. I've even tryed to float
: thinner on top before capping. I hate thowing away $5 worth of varnish
: from every can. Any solutions? Should I transfer to smaller containers?