If you have rust, I would suggest
1) use a wire brush (one mounted on a drill or angle grinder works well) wipe off with a paint friendly cleaner—it is a pain to have to degrease in order to get paints to adhere.
2) apply a rust convertor (your choice of brand—there are lots out there)
3) use a decent primer—the cheaper you use the less well it holds to the metal and final color coats of paint (I like a self etching primer—these hold to metal/rust converted metal and cover coat well) For touch up work I usually use rattle cans. Mostly do to being lazy to mix and clean up paints for the air sprayer. With rattle cans I can be done not counting waiting between coats in the time it takes to prep with the spray gun.