epoxy the car what ever color you want to use, then put on a high build primer, a little guide coating NOT a full coating as that would be a waste of material, block the car smooth to what ever grit you want or upto 400. If you break through on a high spot reprimer and block until flat. sealer is the first coat BEFORE painting, other wise if you use the sealer now you will have to rescuff and reseal before painting. the sealer you can tint with base coat to aid in better coverage of the base coat.
ZINC is the key to metal prepping, either a zinc wash as in metal ready or ospho, this neutralizes the metal so moisture doesnt penetrate the steel and start rusting again. Black has a higher zinc content vs, grey, red, yellow. at least in the RM/BASF line that I know of.