For any type of aluminum trim you need to remove the "bright dip annodizing" first. Sanding is one way, but difficult. The fastest/easiest is a chemical strip. Once stripped you can repair or polish the piece but it will no longer have any protection and will eventually oxidize.

Forget oven cleaners - too weak. Sodium Hydroxide (found in Drano Kitchen Crystals) will do it. I've done it very successfully in a container large enough to submerse the piece. Hot water, dump a bunch of crystals in till dissolved. (Safety disclaimer: Wear goggles, gloves, adequate ventilation as some hydrogen gas will be produced. Proceed at your own risk!!!!! No lectures, please).

Place the piece in the dip. Some small bubbles will form (and a tiny amount of gas). Keep an eye on it. In 20 minutes or less all of the bright dip coating will be gone. Rinse the piece under cold water. You may have to rub off some gray colored "smut" or residue on the surface - it will come off easily. You'll be left with a clean aluminum part ready for polishing.

With all that said, this DIY process is probably not practical on a grille shell. Headlight bezels, straight trim, yes. I made a dip tank for long straight pieces out of vinyl rain gutter with end caps. No matter how you do it, it's a lot of work. Consider sending it out to someone who specializes in this sort of restoration.