You can put as much paint on as you like but if you don't prepare them first, they'll be rusty within weeks. It's why i do my own and not go the powder coating route.
So here's the last set i did last year. Never been on the car, but sat in the garage - which at times during the cold winter gets a lot of moisture - but as you can see still perfect. I'll fit these when the current one's go rusty, but they haven't yet
Whip the old parts out, buy a kit from Big Redd. Knock off any crud with a small hammer if applicable and agitate the rusty bits with a wire brush. Buy some Bilt Hamber Hydrate-80, if you haven't already got some, then put a couple of good coats over the calipers. It'll dry a blackish colour and convert the iron oxides into a metal complex. It'll also leave a barrier to stop any surface moisture restarting the corrosion process.
Nip down to Halfords or go on interweb and buy some VHT paint - colours are a bit limited at Halfords but i like titanium so just mix Black and Silver until i get the right colour.
Brush on a few coats and leave to dry overnight. Put your new parts in and jobs a good one. I used to swap my old rusty calipers for refurbished ones but they never have enough paint on them so just rust through. Powder coating is no better long term as they don't treat the surface prior to coating, effectively sticking paint over metal that's already started corroding.