My wife's '97 Z28 has factory painted white rims. I can't seem to get all of the brake dust off of them espcially on the back. The front side comes out OK. I also have some black dots, which I assumed are melted brake dust that are hard to get out. Any ideas on how to get them real white again? I want to clean them up and then seal them with Klasse, but I want them as clean as possible before I do that.