What I have done is before putting the lid on, put a piece of plastic over the opening (sandwich bag works for small cans), push down a little in the center of the opening and then put the lid on and seal it. If the paint dries between two pieces of metal, it will bond them together really well. Store the cans upside down to prevent air from entering the can (this works for all paint cans). If you have a place where you can do it without initiating a civil war, store an opened can in a refrigerator. I have done this successfully with open cans for over a year.
