I just used some yesterday on a vintage plexi. Have been using it for years, never tried it with sandpaper, never had to. Depends how deep the scratches are i suppose.
As db2 said "Put about a Pea sized dot of polywatch on the top of the crystal and then rub it in a circular motion until the polywatch disappears and the crystal is clear ..."
Rub with some pressure for about 3 minutes.