I had my motorcycle ones done by Redditch Shotblasting. They needed the the pistons, seals and bleed nipples removed. They then plugged the small holes and high temperature taped the bores. Be carefully if it's a two piece (split) calliper as I have found some have seals at the joint. There were no problems in reusing the pistons and seals, just make sure the powder coating hasn't formed a lip at the cylinder opening which will damage the seal.