To keep finishes and paints from skinning over in the container, you need to create some kind of barrier between the oxygen in the can and the finish. You can buy inert gas products to form an invisible barrier, but what if you don't have any on hand? Here?s a quick and inexpensive solution.
In the bottom of a clean half-gallon container, put a teaspoon of baking soda, then pour in a 1/4 cup of vinegar. Swirl the mixture. The bubbling reaction creates carbon dioxide -- an inert gas about 1? times the density of air. (Those measurements yield about 2 quarts of carbon dioxide.)
As the reaction slows, quickly pour the gaseous contents of the container (not the sludgy remnants) into your can of finish and reseal the can. The carbon dioxide replaces the air in the can, providing a protective barrier over the finish.
--Kevin Matocha, Troy, N.Y.