This is important. Paint does not stick very well to these shiny coatings. No need for sandpaper or anything that drastic. Just make sure to remove the coating. The final surface should look dull and flat.
Step 2: Wipe the plastic down with a paper towel to remove all plastic dust and junk.
Step 3: Spray one light coat of Adhesion Promoter or Plastic Primer. You don't need regular primer you need primer designed for plastic or adhesion promoter. ( if you can't find any adhesion promoter, a light coat of hairspray will work in a pinch) Hairspray is basically the same thing chemically. Whatever you use... only use a light coat.
Step 4: Wipe your project lightly one more time with a paper towel to remove any dust.
Prepping you project is really important if you want your paint job to last for a long time. A little prep work goes a long way.