On cleaning "copper" contacts (which may be gold plated): I would first try using a "magic rub" pencil eraser, rubbing it over the contacts. It is very important to use this specific brand, "Magic Rub", it's non-abrasive. These are popular and common and this brand of eraser is carried at all office supply stores (OfficeMax, Staples, Office Depot, etc.) On sanding plastic, DON'T. Instead use automotive rubbing or polishing compound and rub/buff with a soft cloth. (if you are going to paint, rubbing compound is the coarser product). It's possible these will leave a finish so smooth that you no longer need/want to paint.