Steve, the cold won't bother the varnish, but the air trapped in the can will dry out a layer on top...with the result that the good stuff left in the bottom will be thicker than ideal and contaminated with some hardened varnish......solution is to strain the varnish through an old panty hose or paint strainer, and thin it.....good luck...if it were me, i'd just buy a new can, because fixing one bad coat of varnish can be a fair amount of work....