I have powder coated all my off road racing chassis and they held up very well extreme harsh conditions that we but them thru racing the Baja 1000. However on my new chassis for my 30 build I personally painted it with Dupont Imron Elite Top Coat. Everyone cannot believe the look as no orange peel at all, in fact most say "so who powder coated it"
Yep , I sanded it with 320 and painted it . No problem. I did'nt know that there was a powder primer until I was telling the PC guy that I wanted to paint over his PC . He was the one that suggested the epoxy PC .
I PC w epoxy all the time at work (big pipe fittings) Just remember epoxy oxidizes (even in the shade) so put something over it.