Was the tape damaged before you took it off? If not, the scuffs were there at
the factory and are most likely scratches into the plastic. In that case the
gentlest of the Novus polishes is probably your best bet. It will be very
difficult to restore a perfectly smooth surface.
If the tape was damaged, you have to determine if the scuffs are residue from
the tape or scratches in the plastic. If the former, I would use isopropyl
rubbing alcohol on a microfiber cloth before trying anything stronger. Note:
Ethyl alcohol may damage the plastic. Use Isopropyl.
DO NOT use goof off or goo gone on a plastic surface. They can easily melt the
plastic.
Always try the gentlest possible solution first. I don't know PlastiX and so
can't comment on it.