Most powdercoating shops shy away from anything having tight tolerances or extensive machining. They have to be extra careful when taping up the housing. The other problem is how to support an axle housing while in the powdercoating oven - if it is hung wrong, the housing will be warped when you pull it out. Powdercoating an axle housing is usually something they don't bother with, but if your guy will do it just make sure he is careful with it.