What kind of metal? Anodized aluminum? Stainless? Chrome?
When I used to work on multi-million dollar marlin sportfishers believe it or not the solution of choice amongst the fleet was white vinegar on the anodized aluminum. The acetic acid is pretty gentle and does a decent job taking off the crustys if there's not too much corrosive damage to the metal. Collinites cleaner wax was the only thing we used on the stainless bowrails, rubrails, and other hardware. I don't remember the specific formula though.
