I am getting really tired of reaching for my can of polyurethane only to discover that that the only thing inside the can is a semi-solid goopy mess. The problem arises after the initial use of the can. Once oxygen gets into the can the damage begins.
As a chemist, if I were dealing with a reagent in a laboratory setting, I would purge the can after each use with either nitrogen or argon. Unfortunately, I don’t have cylinders of these inert gases laying around. There are products on the market that do just this, but they don’t appear very cost effective to me. An interesting approach that I read about on the net involves adding marbles to the can after each use in order to displace the air from the can. I have also read of woodworkers using ‘dust off’ sprays which are composed of difluoroethane, which should also work to do the job and is readily available and cheap.
What method works for you?