I use Rustoleum Rusty Metal Primer and Rustoleum gloss black on everything which is black on my trucks. I use rattle cans. This paint flows very nicely and ends up smooth as all get out, and holds up very well.
The best part is that touch ups are a breeze, and every hardware store in America carries it.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission