You could probably apply your clear over the paint but I wouldn't. It might not wear any better and complicates repainting. I'd apply several coats of the finish paint and then IF it wears away - sand lightly and apply another coat.
How exactly does one add a hardener to a rattle can??
You don't add it, you buy 2K spraybombs. Example: https://www.eastwood.com/eastwood-s-2k-aerospraytm-ceramic-chassis-black.htmlHow exactly does one add a hardener to a rattle can??