Best convertible top cleaner?

[Seattle Sam]

Hi folks,
I'm looking for the best way to remove black and grey marks from my white convertible top. I have tried scrubbing with various fabric cleaners, including my wife's "special quilt soap" [img]/ubbthreads/images/graemlins/crazy.gif[/img]
but I have not had luck removing these tough stains. I have looked at a couple of products in the local automotove store, but they carry warnings about keeping their stuff away from paint - I don;t really want to use anything that can harm paint.

Anybody want to share their favorite cleaning material or method?

Thanks in advance,
-Seattle Sam