Many dealers use a very acidic cleaner to remove brake dust because it is cheap, quick and easy. Most of the "detailers" I mean that lightly are young kids that spray a gallon of the cleaner standing from about 5 feet away and the overspray and splashes can get on the aluminum running boards and it will stain. Doesn't damage the painted surface of the wheels if it is removed quickly. Maybe this is what has happened. I have seen this before on unpainted aluminum. Ask them not to use an acidic cleaner. Better yet, tell them not to wash the car at all.

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2013 BMW X6M-current
2013 Audi S4-current
2007 BMW X5 4.8i (E70)-sold