Is it brown anodised or brown powder coat?? If it is brown powder coating give it a clean with a quality Auto paint cutting compound and apply a quality auto polish and buff to a shine. If it is anodising be very wary about getting chem's near it...You can chemically cut the anodising right off.. If it is anodising it could be just sun bleached and past it's use by date and the only way to fix it is replacement. Not all solutions to problems come in bottles of chemicals.