Remove the calipers off the car...then remove the dust boots, seals and pistons with an air gun (take care of your fingers and put a piece of wood between your pistons to avoid damaging them). Next to this, soak and clean all the parts with a soft brush in an electric gasoline tub. Bring them to your powder coating shop and ask the guy to sandblast them before they apply the powder coating stuffs. You are all set now...
Note: Aluminum calipers require more precautions than steel calipers because they make a reaction under heat.