I also have a Mineral White X1 and this dust accumulation. I think there is something about the aerodynamics of the X1 that causes road dust (which includes a fair amount of brake dust from other folk) to accumulate all across the back end of the car. Switching from a Subaru Forester, I notice this accumulation is much greater on the BMW. Also, I find I have to use the back wiper at least four or five times more often on the BMW when it rains than I did with the Subaru.
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