If you have any bare magnesium exposed from bead blasting the wheels, you need to prime them with Zinc Chromate primer, available in aerosol spray cans from Aircraft Spruce.
See this thread for an example of what can happen when a wheel is left un-primed or incorrectly primed:
http://pantera.infopop.cc/eve/forums/a/tpc/f/1510042044/m/4420064984
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