the problem with magnesium wheels is that once the surface of the paint is damaged the corrosion rapidly spreads within it, powder coat is likelly to offer very little more than a regular paint, in my experience the best coating for mag is Tagnite (Tag 8200) then 2-3 coats of Rockhard paint (by indestructable paints) they do most colours, but not metalics and is applied in the same way as any regular paint.