The top coat is almost academic. Everyone has their preferences & they're for the most part all different.
It's the prep' that's important and what takes the time. I spent 2 ****ing horrible days under my III with a twisty wire brush thing on me grinder (prolonged use burns out drills quicker, 'parently) wearing thick gloves and eye protectors. Then washed the chassis with solvent, then put on 2 coats of red oxide primer and a gloss black for metal as a top coat. Buggered if I can remember the make of it now, but jet washed orf, me chassis still looks good a couple of years on.
The wire brush will also expose any areas, seams, etc that have rusted thin. I had to put a patch on one area before painting. Looked fine until I went at it with the grinder, so be prepared for a shock or two!