Pouring a layer of paint thinner into a can of polyurethane gel

How do you keep varnish from crusting or drying?

Michael Dresdner has suggested floating a layer of mineral spirits atop thick oil-based varnish or gel urethane to keep oxygen away from the finish so it doesn't form a crust inside the can. My question is, how do you get rid of the mineral spirits when you are ready to use the varnish again? - Philip McGregor

Tim Inman: There have been many such ideas offered for accomplishing the goal of stopping finishes from gelling or hardening in the can as they sit on the shelf. I don't know of any that really work or are long-term effective. Sucking in some air and holding your breath until the oxygen is gone, then blowing that carbon dioxide- and water vapor-rich air into the can just before clamping down the lid quickly is one of the more weird ones. There are commercial products that tout the ability to prevent in-the-can hardening. Usually, they are nitrogen-based and come in some form of an aerosol. The idea is that you puff in some nitrogen and then slam the lid on before it can get out. Here is my better suggestion: Buy smaller amounts. Keep the lid on tight. (If you'll punch some drain holes in the bottom of the can ring where the lid pushes down — on a conventional paint can — the lid can be pushed all the way down and seal better. Once the lid is on, shake the can to coat the inside of the container, forming a sealing paint. Or, buy some smaller cans and re-can the product. Keep the smaller cans full so there isn't air space above the finish. Regardless of what you do, finishes like urethanes will cure in the can over time. They react with water vapor as well as oxygen. Buy just enough to do the job, and buy more later when you have another job. Old varnish has caused me so many questions to answer over the years, and they have caused so many disappointments to the users, too. Get fresh varnish when you need some, and you’ll be happier.

Chris Marshall: I use small glass canning jars for the purpose Tim describes, because I rarely buy finish in containers larger than quart sizes. When those cans are about half or less full, I’ll transfer the varnish to a jar and fill it to the top, then label the metal lid with a permanent marker. This way, once I use up the finish in the jar, I can clean and reuse it again. The glass also makes it easier to see just how much finish remains, and the marker I.D. is easy to wipe off with denatured alcohol.