Matt spray is the best, goes on very thinly. My rear discs were scrap due to the rust inside the drum, nothing to do with wear. A coat of paint would have saved them.
Whule you are doing the discs, give the back splash plates a spray with some rust killer and give them a coat of paint to stop them from rusting away. Worth giving the ABS pickup wheel
a clean with some brake cleaner and a toothbrush.
I had to replace the rear plates as they had rusted around the holes that mount the handbrake shoes. Pig of a job. The front splash plates were both missing, just some
rusty mounting points left. Another pig of a job that I still have to do.