What is primer? Do I have to use it?
YES, YOU MUST USE PRIMER
Primer is a filler and a barrier coat, preparing the plastic to grip the paint, filling scratches and imperfections, as well as neutralizing any incompatibility between the paint and plastic. Automotive plastic primer is the best choice, as it is suitable for any top coat and comes in white or gray. Also available in clear though that has no filling properties. ‘Key’ the plastic by rubbing the whole body with 800 grade wet & Dry paper, then apply up to three coats of primer. Then leave for 24 hours, smooth with 1200 grade paper, then add the top coats.
Also, the primer coat will help you find any small imperfections in the plastic or mold before you continue. As you sand it after the first light coat, the sandpaper will naturally take primer off the high points. These high points must be sanded flush with their surroundings, or else you will polish right through your paint job in these spots!