I wouldn't go near the playfield with an Orbital either, far to easy to damage the playfield. I have used small buffers on the playfield, but you still need to be careful.

Probably better to polish it with the Novus by hand, after using Nifti to clean the playfield first.

Nino has written up a thread about how to get rid of the ingrained dirt if it's really bad here: http://aussiepinball.com/index.php?topic=66.0