This is a stock wheel from a 2003 Toyota 4-Runner. You can see where I've started cleaning it where it shines and other areas not so much. I run my fingernail and the "scum" does not budge. Is this some sort of build-up I can get off with a product or is the alloy wheel damaged and needs to be re-painted/coated? What do you recommend? Thanks for any help you can offer.