[papajam]

The 'bubbling', as it occurs in powdercoated cast parts, is called 'out-gassing'. It occurs when tiny pockets of gas left in the metal from the casting process rise to the surface when heated. The gas will break thru the curing powder and look like bubbles. The way to prevent outgassing from ruining the finish is to pre-outgas the uncoated part first. Just stick it in an oven that's at a higher temperature (450 F) than the cure temp (400 F).
Don't know how safe that would be for a mag wheel though.