Fresh finish: it's in the bag
I started using polyurethane paint a couple of years ago for high-gloss color coats on toys. Unfortunately, the expensive finish cures quickly in a sealed but partially full can. After many unsuccessful attempts at preserving the leftover paint, I struck on this method.
Tuck a zipper-type freezer bag into a glass jar and pour the leftover finish into the bag. Squeeze as much air as possible out of the bag before zipping it tight. Then, leave the bag in the jar to protect it. When you're ready to use the finish again, cut a small opening in a corner of the bag and pour the finish into a container for use.
—Bill Wells, Olympia, Wash.