You know, since the damage is already done and the case is really yellow, maybe you should think about doing something cool and creative with it instead of painting it just plain old white.
Pick a color that would be impressive, like red or a cool indigo or something. Just stay away from glossy paint. Use flat paint. Pull the case apart, remove the logo plate, prime it with some white primer and then apply a few coats of your favorite color and when they are dry hit it with a little gloss coat or dull-coat.
I wish I still had the machine, but, I once had an A500 "parts pile" which essentially consisted of 10 broken A500's with nasty yellow keyboards. One day we made one functioning, but ugly, system with it. I am also a Warhammer gamer, so, I have an airbrush for painting my larger models so we used that to paint the crappy old case in a multi-color pattern. It came out great. I will try and dig up pictures if I can.
- Mike
