Are you kidding? I get several months of use from an 'open' spray can. All it takes is to clean the straw and nozzle with some acetone (carburator cleaner has acetone in it) right after each shot. I particularly like the small $3 cans - about the size of a soda can- you can find from time to time.
Sealing is usually the last thing you do - and you're usually sealing wall penetrations and all manner of other openings at the time. Foam? It's what's at hand.
