A lot of diamond plate comes with a clear coat. Your gonna have to strip that off. Once you get past it, you can use a buffing wheel and some metal polish. I recommend Flitz, its good, but expensive. Run the wheel until the metal turns black. Use a clean terry cloth towel to wipe off the black residue. Your diamond plate can be polished to a high shine, and the Flitz seems to put a protective coat over it. It will take some work, but it sure looks good when its done.