Well I can tell you this, I'm going to test my brakes real good before doing any traffic driving, since I'm completely flushing/bleeding my brake system as well. Paint will not make my brakes fail, they may be underpowered for a little while. Look at the picture of the rotors after, the two front rotors don't even have a full coat of paint on them, you can see the metal through it. It's only a little thicker than when they have a glaze of rust after a rain. As with my aim, well it's spray paint, it's going to get all over the friction surface since you spray from a foot's distance and paint the hat of the rotor. Why do you think when people paint them on the car, they tape off the entire wheel well?
The rust you refer to when the car sits, would only be the friction surface instead of the entire rotor, which I'm fully aware of.