I have no experience painting it, but I would try to find out how much
it moves. I know vinyl siding moves an amazing amount - pieces can
vary by inches depending on the temperature. I don't think the plastic
moves nearly as much, about the same as wood (guess) but I'd still go
with a high quality latex, no primer.
I have used the trim in only a couple of places so far, both bathrooms.
I have a window in the center of my tub, so even with a shower curtain
over the window, I needed to replace the funky wood moulding when I
remodeled. (Should have done it sooner. <shudder>) It has held up
well for the year or so it's been in place. The caulk is holding
tight. Ditto for the basement bathroom floor molding - no caulk there,
I used long, aluminium white siding trim nails to hold it in place.
They have some head to them & the moulding is white so... It worked
out well. I did start the holes with an awl, but they drove fine after
Any voids behind the trim & the stuff cracked much easier than wood, so
you want to watch for that. I certainly would use it rather than wood