There are all sorts of ways to prevent varnish from skinning over or hardening in the can between uses. Here’s the tried-and-true technique I’ve been using for two decades: I pour my quart containers of finish into two clean, half-liter plastic soda bottles and cap them tightly. When the bottles are nearly full, it’s easy to squeeze the air out, but as the bottles empty I flatten them in a vise to expel the air. I’ve never had a bottle break, even when clamping down hard. It’s cheap and effective.