I have done this on several bikes. I have not got hardener in the paint but the clear coat had hardener and was in an aerosol can. However when I go this route I plan on spending at least $100. Not cheap but the paint lasts. Disappointing to do all the work to have chipped up paint from assembly.
I just got the Montana paint the other day and grabbed a can of their Varnish which has a UV protection but I doubt it has a hardener. Going to visit a body shop to see if I can get some mixed up. For the bike I'm building, its running a front derailleur, so last thing I want is for installation of that to ruin my paint job.