The calipers must be stripped of all coating, washed, and dried. Then, they must be heated to 400 deg. for about 40 min or so to "burn off" anything that could contaminate the powder. Next you coat'em and put them back in the oven for 30 minutes after "flowout" (the change from powder to liquid) has started.
Just keep in mind, DOT 3 brake fluid boils at around 400deg. (dry) and 284deg. (wet) "dry" means no water in fluid, and "wet" has 3% or more moisture present in fluid. Since these are used calipers, they will have moisture present in fluid. The only way to completely remove the fluid is to remove the pistons, and flush all passages. If you are careful and put all pistons and seals back were they came out of, you MIGHT be OK. This should help you decide what to do.
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1970 T1 W/MassIVe 2913cc RAT/?EFI? w/direct fire (very soon) and 915 trans
1962 SC 1776cc SP 944NA brakes, 993 wheels
VKG