Any proper polishing compound should work. They do need more work than normal car paint surface but eventually it comes out good. I simply used Autoglym Super resin polish. Then after buffing I coated them with ceramic coating (Nanolex Si3D).

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E39 530dA -02 M-Sport Messing metallic
E53 X5 3.0dA -06 Sport Stratus grey
E70 X5 40d -12 M-Sport Space grey