Storage of Plasti Dip?

[Ed in Tampa]

Press some Saran Wrap down on the surface so any air in jar can't get in direct contact with the fluid. Or fill the air space with an inert gas as some suggested, that is only way to preserve it.
Turning the jar upside down only makes it harden from bottom ( which is now the top) down.

You could transfer to whole thing to squeeze bottle and then Squeeze out all the air.

The problem is not air leaking in or out ( depending on your perspective) but rather the air that is in the space vacated by the product you took out. If you took a little out and closed the container it would probably last forever. But if you took enough out so that air that came into that vacated space was enough to react with the product it would all get hard.

Good luck.