Make sure you have clear coat, otherwise the paint will scratch right off. Also, I'm pretty sure you're talking about psp trim, but please specify. Many things have "trim" that could be painted, not just the psp. I would say any amount of layers over 3 or 4 would be good. But remember to sand between layers.
Also, as far as scraping off the silver, the paint should cover that. But before starting to paint, use some 800 grit sandpaper to go over the trim, because that will prep it for painting.