Painting Info....

I am aware that some have done what you are asking successfully but it is generally not recommended. Certainly no automotive paint manufacturer that I know of suggests it. Quite the contrary. People who work with automotive paints would frown on such a practice. There is too high a risk of either possible wrinkling or greatly retarded curing of the first coat. The first coat would cure by evaporation and oxidation whereas the following coat(s) with hardener would cure by evaporation and chemical reaction of the components. A non-hardened enamel can take several months to fully cure. A hardened enamel cures in a matter of hours or at most a few days. That's the reason that once you add a hardener to an enamel (or urethane) you have to use it in the next few hours. You can't mix it up and put any leftovers back in a can. It will jell and harden by chemical reaction even in a tightly closed container.

Can you do it and get away with it? Possibly, but I wouldn't do it and therefore don't recommend it.

Rod