Most modern paint sticks fine to plastic. Adhesion promoters make the bond stronger, but shouldn't be necessary for the NACA vent.
Good cleaning is key with plastic to get rid of any mold release agents that may be present. Then just scuff with 220 then 400 and prime as any other part. If you prime in advance, you still need to scuff it later to get adhesion from the final primer coat.
Larry
Thanks for the responses, I should have asked a bitter question.
What did you use to prime the outside of the NACA Vent?
Thanks,
Fred