I steam clean everything, wire brush loose rust, wipe it down with paint thinner, brush on Ospho, brush off any residue the next day. Then I prime with Rustoleum rusty metal primer and paint with Gloss black.
The reason for this strategy is that any touch up can be done with paint that Ace Hardware always has in stock. Rustoleum black paint flows and levels very nicely out of a rattle can.
Carl

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