C38Coupe, You asked about using a 2K Clear over regular enamel. It was discussed very briefly a while back in this thread. I believe someone used Nasson (by Dupont)clear over rustoleum. If this is the plan, using some hardener in the rustoleum is a good idea. The hardener will add a significant amount of chemical resistance to the paint. If I recall correctly, it increases the chemical resistance by like 40%. This increase in chemical resistance may enable the rustoleum to withstand the strong chemicals used in 2K clearcoats.

Adding hardener to the Rustoleum is probably not essential if the correct clear is used. I've read that there are some regular basecoats (for base / clear systems)which do not use any hardener in the base. So using a clear coat that's compatable with a non-hardened base may work fine with regular enamel. (I've not personally tried it though)

For what it's worth, there is a hardener specifically intended for regular enamel paint. I've bought it at agricultural supply places.

Safety precautions must be followed with any hardener. The isocyanates used in hardener are very dangerous. Most paint manufactures recommend a supplied air respirator when spraying these chemicals. A few respirators on the market can remove these chemicals, but there is no way to know when the respirator is saturated as the isocyanates have no odor. If a person is considering using a respirator, get one from an automotive paint supply shop. There are charcoal respirators available at many hardware stores which will remove the smell of the paint chemicals, but wil NOT remove the isocyanates.

Lastly, I'd suggest counting up the cost of the clear, harder, reducer, and enamel. It probably would not cost much more to spray some regular automotive single stage or base clear. (PPG's Omni or Duponts Nasson lines are decent quality with reasonable prices. Of couse there are Kirker, SPI, Transtar, Western, and other discount brands. SPI has a decent reputation and their single stage black was around $100 a gallon a while back.

I'm not against rolling paint. In some situations, this may be the only practical option. But if a person is going to spray paint, it makes sense to consider regular automotive paint.