I have painted many cases with Spray cans. The trick is PATIENCE! Do not try to cover everything and get the perfect paint job in one pass, you will get runs and all sorts of ugliness this way. It WILL take you three to four or possibly more passes if you are doing it right. I apply the first coat, as even as possible (can still see some of the base color through it) Let it dry for 5-10 minutes, hit it with the second coat. (Most of it is ussually painted by now) let it dry another 5-10 minutes, go back for a thrid coat, spraying evenly across the whole case or panel still. After the third coat is dry, I inspect it and if there is still some base color showing, I'll hit it yet again. MAKE SURE that you spray from different angles around the case in order to make sure one corner is lighter than another for example. Don't get greedy with the paint. If you paint too much in one spot it will show in the final result. Nice even strokes, multiple coats, and you'll be ok. It all comes down to experience. Tag a few practice pieces of something non-important first if you have never done it before. Get the hang/rhythm of spraying down before you hit your final target with it.
Edit: for plexiglass panels, I'm assuming you've got some vinyl or somethign laid down that you are going to peel away after you lay down the paint coat... Just do exactly as I described above, nice easy layers, don't let the paint dry all the way though between layers. You want it pliable when you lift off your vinyl/masking materials. When peeling the mask off, fold it almost in half and pull horizontal to the paint, NOT straigh up. You might even keep an X-acto knife handy to help cut the paint if it tries to lft with your mask.
Have fun!
