Repainting engine cover - do I need to prime now?

[Mossyoakglock]

Last week I painted my engine cover but don't really like the way it turned out. Plus I found a shade of gray that closer matches the truck so I want to repaint for that reason as well.

I am going to sand it down as much as I can but I'm curious if I need to prime it first. After sanding, should I use primer or can I use adhesion promoter then spray the color? On the subject of primers, which should I use? There is the standard gray filler-primer but they also make a white primer specifically for plastics. Being that I'm using a dark silver/charcoal color, would a white or gray primer be better?

If I use the plastic specific primer do I need to use an adhesion promoter or should I use it regardless which primer I use?

My thoughts are: sand/rough up surface, adhesion promoter, prime (either standard gray filler or plastic specific white), lightly resand, paint desired color, clear.

What are the steps that you would use to repaint the engine cover?