Some paints will say you don't have to but its been my experience that the best quality paint jobs against chipping need a good scuffing up with either sand paper or a sand blaster, even with self etching primers. Apply the self etching primer for the bare metal parts, you can pick these up at your local autobody repair store and even some hardware stores will have it as well. I get my etching from NAPA paint stores, its a higher quality automotive self etching and flat black form Dupont. Here they are, the two on the left:
They are a bit more expensive than most self etchers and flat back but they are very durable compared to most I've used and hold up well to bumps and bangs.
That's about it for flat black, its probably the easiest and most forgiving of paints to use on a case, once you get into the glosses every little imperfection will show. As mentioned, many light coats instead of a thick ones. A tack cloth is also good to pick up, clean the case down of any lint etc... just prior to painting.
Also, as mentioned, disassemble as much of the case as possible, you should be able to really take it apart. Depending on the case i'll sometimes even drill out the rivets to disassemble even further.
You should be able to break it down to just the primary casing, like so:
That will yield the best results so that every little crook and crany is covered....also minimizes the taping.
Ok, final tip, read the instructions and safety label on the cans, some of these paints are pretty nasty so made sure if you need an organics respirator you get one....an N95 dust mask will not protect you.