As above, though I tend to use a fine wire wool to get the surface to a very matte/keyed finnish before painting. As for the paint its self Folia tec caliper paint is the best you can buy imo, but it's easier to get a good finish with the spray can stuff.
If the discs are very badly rusted it may well be worth taking the discs off the car and getting a machine
shop to skim them down to clean metal.