Cubic, here is my two cents worth on case painting.
I usually sand the metal parts of my case lightly to rough them up a bit. After I sand it I wash it with warm water. I use a warm washcloth on the top since it is usually rivited in. Then dry your parts and use air to make sure any towel lint is gone. Do not touch your parts with your bare hands because your hands will leave oil that will mess up the paint. I prefer Krylon brand paint. I usually give the metal parts several light coats of grey primer. Sand out any imperfections and wash the parts between coats if necessary.
Apply several light coats of the color coat, several light coats are better then one thick one. The best advice I can give you is take your time. I have allways used spray paint on my plastic parts, some people prefer using vinyl die. I have not used the dye yet but I think that will give you better reults.
If you have any more questions I will be happy to help.