What a coincidence! I just did my roof with the same issue today in a 49 Chevy. What I did was DA with an 80 grit to clean up the surface rust. Then I blew it with air and cleaned with a prep cleaner. I used POR15 with a brush in tight areas and a 6" roller on a 24" handle. Worked slicker than dog poop. The roller is very fast and the por is very level, looks good and and went on easily. Had a hat on in case, but no drips that I know of.
I have some tie coat left and I am going to use that up and then spray on a coat of color. Ofcourse I went the cheap route on areas that will not be seen, roof, interior panels etc and used a cheaper enamel with a hardner. You will probably laugh, but Valspar implement enamel with hardener is pretty tough and looks surprisingly good. Six bucks a quart plus hardener six bucks for a pint.
If you wanna go expensive in a spray, I guess 3M Schutz may be a choice for an under coating.