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,
though.

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
that.

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
outside.

Jim