First thing you need to do is get all the years of paint, wax and armor off of them. Use laquer thiner for the paint. WILL NOT EAT PLASTIC. use Enammel Reducer for just wax.
Wipe the whole thing down, about 2-3 times, with enamel reducer.
Then sand with a scotch bright pad.(medium coarse).
Wipe it down again after sanding, 2 times.
I then used a flexible plastic primer on mine. Start with very light coats. It the primer has a "fish eye" look to it, stop and sand the area again and wipe it clean. Fish eye look= still wax on the plastic.
Give them 3-4 good coats.
Sand again to remove any dirt or dust particles.
Wipe clean again with Enamel Reducer.
Paint. I used black spray bomb on mine, turned out great.
Hope this helps