After reading all the post on this thread, it is easy to see how everyone has gotten hooked on powder coating as I have myself for several years now. I bought the cheap Eastwood single voltage unit and powder coated my OEM manual lockout hubs after sandblasting and cleaning with Acetone as the first pieces. I couldn't believe how quick and easy they had cured to a hard finish and could be installed immediately afterwards, no waiting for paint to cure. Never painted anything again after that, unless it had plastic or some other material that would melt while curing in the oven, that I got off a roadside curb from someone who threw it away during a kitchen remodeling project.
With that being said, powder coating is somewhat of a science as I have learned from trial and error and reading many articles and forums on the subject. I need to upgrade my gun to dual voltage!
Powder coating different thicknesses of metal, aluminum, cast iron will take different heat ranges and time for curing as well as the type of powder being applied. A Infrared Digital Thermometer is a tool that everyone should have for powder coating. The surface temperature of the piece being powder coated must be the same as the curing temperature of the powder and monitored to insure the powder is not under or over cured. You can actually change the color of powder if over heated on thin metals.
To powder coat disc brake calipers, you need long periods of heat because it takes much longer to get the cast iron hot enough for the powder to start to flow and cure. Probably around 40 minutes at 425°F or when the surface temperature is the same as the curing temperature of the powder. The powder won't adhere well and may scratch off easily if not heated enough for the powder to cure completely. More importantly, the cast iron part should be preheated beforehand at about 450-500°F to burn off any impurity's like oil and and other foreign chemicals that may have been absorbed by the casting. Without first burning off the piece to be powdered, anything that is cast, you will risk getting fish eyes or gas outs. Sometimes they cannot be avoided though.
Take out all rubber seals and anything you don't want powder coated on the caliper, including the pistons. Use silicone plugs to plug all the holes for brake bleeder screws, mounting holes etc. Use a lid, disc, tape or some other means to block powder from entering the cylinder bore of the piston. You don't want to get powder in the inside of the bore and then have it cure because it will be next to impossible to remove. If you accidentally get powder in the bore, use a damp cloth or small brush sponge to wipe the powder away before putting it in the oven. Remember, its not paint you are working with that can be cleaned off with paint thinner or mineral spirits.
Some photos here of powder coated Jeep CJ calipers.... it can be done and heat from the friction generated from applying the brakes has no effect on the powder finish as some people may fear.