I've been using (still) the hard oil based paints, and have had no problems storing partial cans. You do want to get a good seal on the lid - turning the can over once to 'paint' the joint between the can and the lid seems to help. I've never had the bottom paint skin over the way varnish does. What I do find is that the pigments settle and will form a pretty solid cake in the bottom. But this will happen even if the can hasn't been opened. You need a good power mixer to turn the two phases back into paint, but this is a lot less messy if the can is only half full.