If you are talking about the plastic trim on your doors and bumpers you do it exactly the same as metal but:
If you only want it to last a short while by all means cut corners.
There are different ways to prepare plastic and I won't go into all the details but these things are a must:
1. Absolutely clean of dirt, grease, mold release agents, etc... If it's not clean why bother with any other steps.
2. Bare plastic needs some type of plastic adhesion promoter.
3. Flexible plastic parts need a flex additive. It's not cheap.
If someone is cutting corners and giving you a smoking deal on painting plastic you can bet they aren't using adhesion promoter and flex agent. I would think they aren't using a sealer as well.
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