Polyurethane Shelf Life
Article - December 29, 2008
This article was inspired by a question from Terri:
I make signs out of wood and last Spring I purchased a gallon of Helmsman Spar Urethane since I go through quite a bit of it (and absolutely love it). I had used a little more than a 1/4 of the gallon then stored it for the summer in the basement. Its a living basement therefore, the temperature was generally around 65-70 degrees through out the summer. When I opened it this fall to start using again, it had a hard coat over the top and about 4 inches of gel under that. I don’t know if the can wasn’t closed tightly or what happened. I was able to break through the hard coat and tried to mix it all up as best I could but now I have small ‘chunks’ of the gel that gets on my wood work. Is there anything I can do to eliminate this? Should I stir it with a drill attachment paint stirrer or would that cause more problems? I would really hate to have to throw away 3/4 of a gallon so would appreciate any help you could send my way.
Unfortunately my friend, your Urethane is half way into the trash. A skin and chunks are signs that the finish has already started curing in the can. Varnishes cure via oxidation, which means air is your worst enemy. So either the can wasn’t sealed well enough, or the little bit of air that was trapped in the can was enough to keep the curing process going. The key is to put the varnish in a container that has little to no room for air. Or, you can use some Bloxygen. Its an aerosol can full of inert gas that creates an oxygen-free pillow on top of your finish, protecting it from oxidation. Another option is to fill the can with marbles or some other heavy item so the liquid level is raised to the top. But sad to say, this is all for prevention. Once the finish starts curing, as it has in your case, its best to just throw it away. But, if you are feeling especially frugal, feel free to strain out the chunks and test the remaining finish on scrap. If it cures properly and seems to bind strongly to the wood, you can probably get another use out of it. In most cases though, I don’t take any chances once the finish starts curing in the can.