It isn't necessisary to sand to bare metal when you are painting a case (or anything) but it won't hurt, either. Remember, at some point it was bare metal and the paint you have now stuck to it somehow.
The rules for painting are the same regardless if you are at bare metal or if you leave some of the older coats on. Sand with medium grit paper to knock down the old paint and get a smooth surface. Then, before you shoot your first coat of primer, go over the surface with coarse grit sandpaper.
You do this to improve the grip of the primer on the surface. Now, why do you need to use primer? Primer can effect the tint of your final color, a red primer will create a slightly brighter color than if you use gray primer. But the big reason is because primer is like the bread in a sandwich: Its not what you went to the fridge for, but it does a great job of holding everything together. Good primer etches the metal and bonds the primer to the surface, while being porous enough to allow the top coat of paint to bond to it.
Primer is essential if you are working with bare metal. If you've only knocked a few layers of paint off your case it can be optional. But be warned that lighter shades of paint, like that phat tangerine orange you've been eyeing at the Voodoo site, magnify any faults in the top coats like orange peel and runs. And primer is one of your better defenses against that.
One last tip: If you are serious about getting a good homemade paint job, skip the Rust Oleum display at WalMart and find a good local paint jobber. These are the guys who sell paint to the body and paint shops in your area, and most will mix up rattle cans of
high quality paint in any color you want. It will cost more, up fifteen dollars a can, but its a difference you will be able to see.
Oh, about aluminum: The difference is in the primer. Aluminum demands a special primer, it has to do with the microscopic etching I mentioned earlier. But aluminum primer is widely available, and once you get the primer on you use laquer or enamel like any other paint job.
BHD