Paint will skin in a can from moisture vapor in the air, so you must isolate the paint from the air.
Inert gases, such as nitrogen and argon are great, but most don't have them in their shop.
Propane, MAPP and acetylene are common in most shops, although not inert, they work just fine.
They are heavier then air, so they collect on top of the paint, separating it from any air trapped in the can when you close it up.
Simply flow some gas directly into the can, then pound on the lid.
It's that simple.
I've opened paint after a few years, using this treatment and it works like a charm.
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