start MILD then go coarser/heavier if you aren't seeing any results. I'd choose either a very fine scratchy pad (3M Scotchbrite gray or white) and use WD40 as step one...then go to bronze wool and WD40, then try an acid (I'd use Phosphoric first, then Oxalic), and as a last choice a very fine (0000) steel wool with a LIGHT touch and the above solvents/acids. You can always go scratchier, but can't go back to polish out those deep scratches without risking the plating. YRMV