Toothpaste!
Tried it yesterday after removing the moulding line from Tamiya's F-16 non-smoked canopy. Sanded with grade 1000 paper than polished with a cloth and whitening toothpaste (Aquafresh triple protection). The result is satisfying enough, but you have to insist on polishing several times!

Yes, any non gel type toothpaste makes a nice,mild polishing compund. also good for filling smal holes in resin or apartment walls.
Rick