If you use Krylon Fusion paints it has an adhesion promoter mixed in so you shouldn't need to prime it. You will still need to key the surface to get best results though.
Like Lozart said - the most important step is sanding with a semi-fine grit to break up the finish on the plastic before spraying. That said, I prime with their grey first as I've found I can prime then sand again to remove and slight bumps etc. before the coat.
Thanks guys! I just got the standard black as OD was impossible to find, so bought can of krylon OD but didn't get primer so will get some, not sure if I'm going to replicate the OD version or just do multicam or something similar!
