Q. Can brass can be powder coated successfully? Also, can you powder coat over rust, or must it be removed or treated first? –M.B.
A. Yes, you can powder coat over brass. However, its surface is soft and porous, and, therefore, hard to get completely clean. The best way to clean brass is with an ultrasonic system, where mechanical agitation helps work the soils out of the pores more effectively than typical spray systems. The material may outgas and create some pinholes if it is not perfectly clean. To help avoid outgassing, the coating should be cured at a temperature in the lower end of the cure curve.
Rust can be initially hidden by powder, but it is not a good idea. Rust under a coating will set up a low-pressure zone, and moisture can easily penetrate the coating and be absorbed by the rust. The additional moisture will cause more corrosion, the coating will swell, and it will break off the surface. Rust instead should be removed, unless you have no expectation or need for the coating to last for any appreciable length of time.