Find a 6" variable speed grinder. You will need a wire wheel, several felt wheels and polishing compounds.
Use the wire wheel to remove clear coat, oxide, heavy scratches......
move to a felt wheel with a course compound.
Note: One felt wheel = One polishing compound. Don't mix 'em up. Use a sharpie to label the wheels to match the compound.
move up to the next finer compound......rinse lather repeat until you have the shine you want.
I finish off with an automotive clear coat from a rattle can.
Loads of websites for How To on polishing.