I use a Zaino clay bar on the hard to remove stuff. Works like magic! I also used the Mr. Clean Magic Eraser Wheel and Tire, and it is an amazing and fast way to clean off a couple of weeks worth of brake dusk (it doesn't get rid of the hardened caked on stuff).

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-'13 E93 M3 Cab, Interlagos Blue, Carbon Black leather, DCT, all options, and fun, endless fun!
-'13 X5 35i M-Sport, Carbon Black & Cinnamon, Dinan Stage 2!
-'15 Toyota Sienna Limited Premium AWD, Silver & Grey, Nav & BluRay, and less lame than the 2011 it replaced
-'07 Mini Cab S, Laser Blue, 6sp, endless fun that ended (sold)
-'04 X5 4.4i, loaded, loved, modded and sold