For a beater/driver using the gal. sheet metal,a good solvent base grease & wax remover cleaning and a good etch primer. We use this to paint gal.chimney caps and it's the easiest way. Vinegar is a complete joke,unless you got a vat to soak it in for a couple days.
If you want the "last forever" fix. Clean with the G&WR and scuff it with a DA & 80 gt. then clean again and epoxy.
Really cheap out would be to use paint grip sheet metal which already has a wash primer on the gal.