In summary of the links Mr Gov posted here's pretty much step by step to have a effective painting experience.
1) Go to your local hardware store.
2) Pick out what ever color spray paint and a clear you like (make sure its for use on metal, don't buy plastic paint)
3) Look on that same shelf and pick up some primer from the same manufacturer (self-etching is a good idea if they have it)
4) Disassemble the case as best as you can. If you don't feel comfortable removing rivets, at least take it down to them.
5) Prep the surface with some 200 -> 500 grit sandpaper.
6) Clean off the surface
7) Start with the primer and do VERY VERY light coats. The first coat will barely cover the case, second you should be mostly covered, third should only see primer. Then from there maybe hit it with one or two more coats of primer. Lightly sanding with 1000 grit if you get some excess run off.
8) Let it sit and have the primer set in.
9) Repeat the exact same process for the color. All light dusting coats.
10) Let this cure at least over night before you go on to clear.
11) Repeat paint process with clear.
12) Once you are at your desired coverage, wet sand it with 1000, 1500, and 2000 grit.
13) Let her cure.
14) Enjoy