In my experience, if the part is anodized, you can polish it by hand with a rag and some auto polish. It won't remove scratches, just clean the surface, and there will be no black residue on the cloth. If it isn't anodized and has scratches or marks, you could use some very fine sandpaper (#600/#1000) and then auto polish. But if you want to remove scratches or pitting on an anodized part, you must remove all the anodizing first. No problem with that, except you should polish the part frequently afterwards because it'll be prone to spots, e.g. if it gets wet.