This article was inspired by a question from Anthony. He writes:
Is it normal for a mixture of poly and mineral spirits to become like a jelly if its sitting around for about a month or two? I made a half and mixture and when I went to use it the other day it was useless. I guess it would be smarter to make as much as needed rather than making a larger batch?
Hey Anthony. When an oil-based finish turns to gel or starts to solidify in any way, that means its starting to cure in the can. This is a non-reversible process so its a good indicator that the finish should be disposed of. Oil-based finishes cure by oxidation, so the more fresh air the finish sees, the faster it will cure. There are a few things you can do to slow down this process and make your finish last longer, and they all involve minimizing the finishes exposure to oxygen.
Pour the finish you need into a cheap plastic food container, then put the lid right back on the can. This not only makes the finish last longer, but it prevents all kind of crap from getting back into the original supply. I find the best place to buy these is at the Dollar Store. They don’t need to be real high quality because you’ll be throwing them away sooner than later. So don’t buy the good stuff.
Some cans are thin enough that you can give them a little squeeze, and they will bend out of shape. Use this to your advantage. When you pour out some finish, carefully squeeze the can until the finish is near the lid, then pop the lid back on. If the liquid is near the lid, there just isn’t any room for oxygen to get into the can.
An old-school trick I heard about but never actually tried involves using marbles to raise the level of the finish in a can. Just drop individual marbles into a can of finish until the level of the liquid is near the top. When the finish is used up, clean the marbles with mineral spirits and reuse them later.
Bloxygen is a very cool product that is quite effective at extending the life of your finishes. Its a spray can full of inert gas, which means its non-reactive, non-toxic and non-flammable. For you science nerds, the gas is argon, which is heavier than air. So a quick blast of the stuff into a can of finish will displace the air and create a bed of gas that won’t accelerate the curing of your finish.
I would love to hear some of your suggestions for extending the shelf life of finishes!