I run a small local custom paint shop that specializes in pinstriping and airbrush work on Harleys and Streetrods, but a couple of years ago, I began painting odd-ball stuff just for 'cool' points and because I was ending up with a little bit of this color and a little bit of that color. I would spray bowling pins, machined aluminum panels, golf-balls, whatever I had lying around just to show colors and to have a background to do murals on. I was at a show in Topeka when a young man asked me about doing some custom work on a desktop computer case. I didn't know about it at the time, but he turned me on to www.bit-tech.net
where I saw the obvious artwork that was going on melded together with some serious (at the time) electronics engineering. I took my own computer case apart before starting on his to see just what it would take to get a quality finish on one, and although I haven't done any for customers since (haven't really been approached by it), I have done a few for friends and family since then. Here is how I prep and spray the boxes:
Some of the cases have logos or lettering that sticks up and will need to be removed. I use a pocketknife to get underneath of them and pry them up. Try to not scar the surface of hte case when prying. If you scar the surface, you'll have more bodywork to do. After the logo comes up, I use 3M Liquid Adhesive Remover from the auto-parts store. Just use the brush that comes with the adhesive remover can to soak whatever adhesive is left behind, then wipe it off with a paper towel.
I used a Scotchbrite wheel to remove the factory finish. I'm not sure exactly what the material is, but it seems harder than paint, or it may possibly be baked on enamel. Anyways, the Scotchbrite wheels look like two peices of the scrubber material that your wife uses to clean the dishes mashed together with an arbor that fits a regular cordless drill chuck coming out the end. You can pick them up at any automotive store that carries sandpaper for about $8. I strippped everything that is metal or delicate. I don't strip the drive faces or any buttons/lights with the cordless drill.
I then take another peice of scotchbrite that isn't attached to an arbor and I lightly scuff the faces of the drive bays and the buttons with this. I pay particular attention to not scar the plastic faces of any lights.
Some cases come with a flat-steel face. Some cases come with a factory 'texture' in the steel/aluminum. Most of the ones that I've done have been flat, but I found 2 that had textures. If you wanted to, you could use autobody techniques to fill and sand the texture to create a flat surface, but I just left the texture and it looked REALLY good with a light-colored pearl or metallic.
After all the lettering and items are removed, and the surface is scuffed, I use 600 grit wet/dry sandpaper to sand the large open areas by hand to get them uniformly smooth. The scotchbrite on a wheel sometimes puts 'cuts' and 'grooves' in the aluminum cases that can show up in your final paint. Keep the sandpaper wet and just sand until the surface looks uniform.
I then spray the case white. It doesn't matter if my final color is going to be black, I always put down a basecoat of white. It helps any colors that I'm going to use besides black look brighter and brings the color out more. I use automotive urethane paint out of an automotive detail gun (it's like a full-sized gun that you shoot a car with, but it's about 1/3 the size for shooting fenders/doors or just for touch-up work). I spray a basecolor, then a mid-coat "Kandy" color, then whatever artwork I'm putting on (pinstriping/airbrush murals) then my top "Clear Coat".
If you want to, or you dont have the tools to spray multi-coat systems, you can achive very nice results using spray-cans with enamel finishes. Gloss Enamels don't require a mid coat or a clear coat. They dry with a glossy finish.
After your topcoat of clear or your last coat of enamel has dries completely, you can sand the surface smooth and buff just like automotive bodywork, or you can just leave it alone.
Hopefully you can use some of this.