Nice thread, but the guy looks rednecky. I wouldn't trust him.
This is a nice thought, but lets break it down into pros and cons of sorts. If you want to pull this off, you'll need some info.
Painting plastic isn't really any different than painting metal; not in the basics anyway. The problem that arrises when doing so is that some sort of the mix reacts with the plastic and melts it. Ever pour gas into a styrofoam cup? Napalm. (boy, that'll lighten up the NSA's switchboard). Paint of any type uses pigment and a thinning agent. Pigment is just material, and the agent is a solvent. This solvent may, or may not screw up plastic depending on what it actually is. Reducer may not melt one plastic, but thinner may. And this even goes into more variables as you go from brand to brand.
The good thing is that many cars now have plastic (I will never ever type that sentence again!) ABS is widely used, and I've painted more than my fair share of plastic on cars with no effects at all. But a cheap plastic PC case by use cheap plastic. Makes sense to me. This stuff isn't resistant to anything particular. It's just plastic. But you could still do cheap plastic if you wanted to take some of these thoughts in mind.
What melts will harden again once cured. Even that melted styrofoam cup will harden again once the gas evaporates out. If you use very thin coats (and should anyway) of the base (color), then you'll keep the melting to a minimum. Once this layer is built up to a solid base, then it's not plastic anymore really...it's paint. You can then shoot the clear onto the paint, and as long as you don't cake it on, it won't bleed past the base and back into the plastic. Even rattle canning can get the same result.
Now all the bad stuff. Paint sticks to metal by getting into the microscopic pores of the surface. This is why prepping is preached so strongly. The job is only as good as the prep! So, if you sand, etch, tack, clean, etc...really well, then the paint will stick really well. I have no idea how to prep plastic. To get into the pores of the plastic, you'll have to melt it in theory. I guess that sounds like what would happen.
I've painted a cheap case with paint for R/C cars and thought it turned out very well. In the long run though, it's flaking off pretty easily. It never really bonded to anything but itself, making it's own layer. Once you break the bond between its own links, nothing else holds it onto the case; so it just falls off.
And with automotive paint, it comes in cans of raw material. How would you apply it? You could easily get into painting a $20 case with $200 worth of hardware and supplies. Krylon is starting to sound really good here. Even a cheap Airbrush kit or cheap gun ($50) still needs an air supply ($50). And the paint itself isn't cheap. These paints are mixed, and many don't like to mix anything smaller than a 1/2 quart, if that. It's hard for them to mix according to their ratio sheet, and it's hard for you to mix right with your mix cup. Then you get into the big can of $50 clearcoat, reducer, hardener (you can't buy this stuff in small quantities because it's for big cars)...it's getting out of line for a simple paintjob on something as small as a case.
For me, I have paint leftover from a lot of past jobs. I got this habit from my dad that had a wall of paint for me to play with growing up. I have no idea why he kept it, but he did...and still does. He's got paint going back to a job he may have done in the early 80's, and I'm sure it's junk by now, but he still has it. I do the same, so I have no problem with using some Orange left over, or some blue...whatever. But I still wouldn't do something from scratch with new material unless it was for profit or someone else paid for it. In the end, I'd have too much left over to do a couple cases.
My hopes is that soon I could do this for folks. Someone gives me a link to a case they want done, we hash out designs, I order and paint the thing and send it out. I need other things to happen in my life right now to get this right and in some form to make it work, so it's still on hold. But I'm trying to get a real shop together for bikes, and painting cases on the side would be a great little branch-off. This may get squashed, but still consider the plan in motion. I'm learning more about airbrushing so I could do some old-game-type theme for another member here...I'm set to lose a bit, so I'm prepping to pay up