One part or two? Unmixed two part should keep a long time. If the activator has been mixed, you're screwed. Throw it away.
One part paint
will keep fairly well if there's no head
space in the can. An old timer trick is to take up the space with clean gravel, marbles, or anything else inert. A more modern method is to fill the space with CO2 if you have a source of it (not a fire extinguisher!) Just exhaling into the can will display some O2 and replace with CO2, but there's still a lot of O2 in your breath.