I've used it and it dried at least as hard as vintage enamel. We do have 90+ for weeks on end here during the summer though. I waited like 6 weeks. If it is still soft, it isn't dry, and/or your coats were too thick. It might respond to baking, I dunno. That's what the bike factories did.

FWIW I don't really like the "pro" clear coat, and prefer the regular rustoleum clear. Probably best results if you use a modern auto clear.

BTW, the rattle can is irrelevant. Can be good finish or garbage inside. Rattle can paint is usually junk, but it isn't because of the can. So there's an association but it's a false equivalence. There are good finishes available in cans, and some stores will put finish in a can for you.

To get a pro finish out of a can you will have to learn to rub it out though. Not gonna happen "off the gun." Plenty of tutorials on the web.