My dad told me of an old home remedy of taking a few paper towels, soaking them in Cola and placing them on corroded wheels, bumpers, rocker panels, etc. I guess after letting it sit for a little you would just wash off any residue.
It sounds crazy, but my dad comes from the old school where everything sounds crazy but works incredibly well.
Some products like never dull work very well. If they are in real bad shape steel wool or a wet sand, and black, then white rouge polishing compound will polish them up very well. I have made Al look like Chrome with enough time. Afterwards you need a product that seals the surface, products like zoop seal, britemax metal polish and sheen geinie all will protect "seal" the surface. I dont know if never dull will seal, but it does work well.