vibratory polishing of magnesium die-castings

I have problems in the vibratory polishing of magnesium die-castings. I am using aluminum oxide grits and in caustic solution for finishing, athough the surface looks bright, but the salt spray result is only couple hours. Looking over the SEM, the surface is full of aluminum oxide particles presumably due to the falling off from the grits and stick ( embedded ) on the surface which may cause galvanic corrosion with the matrix magnesium surface. Any advices would be very much appreciated.