I have ran into the same problem on my Enkei RPM2 wheels on my 300zx (running our Akebono Sport brake conversion). I ran XP10 and XP9 pads on the car and drove on the street. The dust basically fused to the wheels after not washing it and started to develop rust on aluminum wheels
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I have yet to find a chemical to break the dust from wheels or my calipers once it basically turns into cement.