Fill your partial cans with argon from your tig torch, or have a can of bloxygen on the shelf.
The 75/25 for your wire welder will work to, a lot better than leaving air in there, but it wont work quite as well the argon.
It does not matter what brand or type of paint you have, the oxygen and moisture present in the air will ruin the paint over time.
I always shot a bit of argon into any partial cans, especially activator.
bloxygen