I haven't done a whole lot of buffing to plastics in quite a while.
If i were doing this, i would just test using various polishes on a scrap piece of molding. if that doesn't work out, i go the sandpaper route if you can find 2000 grit readily available at your local hardware store. Your best way to combat this with sandpaper is to do a dry run first. then wet-sand using the same final grit twice. Once with just bare water and the second time using waster and dishwashing detergent. Then polish.