This is an experiment in hand painting a rear quarter bumper panel (not mine) to discover what works, what's possible and what's not so good.
To date...
Two coats of 1K primer mixed with 10% thinners. 500-800 grit sand
Polo Red 6802 2K 2:1 with hardener and 10% 2K thinners. Two coats to date with 800 grit sand.