no one has ever said you have to spray paint. Just get some cheap epoxy like Napa/Sherwin Williams (yes there exactly the same thing). Or even use the self etch primer.Then mix it without reducer and brush it on. Leaves horrible brush strokes but your going to sand it later for the following primer surfacer.As for Rustoleum it is a Alkaloid based paint and all tho its done I would not spray epoxy or eurethane over it. My experience is you can spray polyester over anything and it will lock it in(even tho they recommend only spraying it over bare metal or fillers). Also I like to have just the area I am working on blasted then send it back after the repair and have the whole thing blasted(particularly the weld). I don't blast the outer sheetmetal unless its rusty instead I use a sander to strip it.