If you are painting bare plastic, you will need special epoxy primer, this grips well and won't fall off as the panels flex. Then you use yor metallic base colour as normal (you can't mix plasticiser with metallic) and then you mix a small amount of plasticiser with the clearcoat. If you are spraying over the existing paint after keying it with 400 wet & dry, then you won't need epoxy primer. Standard 2-pack will do fine, or high build cellulose for paint with fine scratches. Use the plasticiser with the clear coat.