Say you mix up 30 oz of paint and think that is enough to cover two coats and find out that you are going to be 10oz short. Do simply mix up a new batch and add it to the paint already in the gun, or avoid mixing new and old? With flash times the paint in the gun could be 20 minutes "old"

what kind of paint are you talking about ?

basecoat ? yes youre fine as long as youre mixing a amount that meets the minimum requirements

something with hardener ? rinse the gun.

I am using a Urethane 2 part with 4 to 1 ratio with reducer. Hardener (called activator) is "optional"