When I painted 45 I sanded the corrosion areas both aluminium and steel, wiped with acetone then dabbed por15 over with a brush. Once dry a light flat back then a spray coat of super etch (etching filler primer good for tying to existing paint). A dab of filler here and there, more filler primer and 2k top coat. I did that in 2009, beat it into trees and swap in Russia and been off roading it ever since. It's held up well some has come back now but most the corrosion now is new not the existing.
Mike