There are paint places that can put your colour in a can, I think I saw somewhere that one of the auto supply chain stores does it. Pressure packs have a high amount of thinner, around 30%, so coats tend to be thinner than what you can do with a spray gun and scratch more easily. I use Wattyl Rust Kill Epoxy enamel thinned 10% in my spray gun and am pretty happy with the result. If the metal is clean or only lightly surface rusted you don't need a primer but they also have a three step process for more heavily rusted metal. If the paint isn't flaky or loose a light sand will do the job to give the new paint something to bond in to. If it is bad, a poly strip disc on an angle grinder will remove the paint without harming the metal or you could get a sand blaster attachment for your compressor. I got one from Supercheap for $20 that works great on small jobs.
