1.   Use an abrasive pad for stubborn stains.

2.   A. The abrasive pad you used was apparently too coarse and cut through the surface finish.

3.   However, I recently had to scrub off several black heel marks with an abrasive pad and this took the shiny finish off in those places.

4.   In extreme cases stubborn stains can even be removed by using an abrasive type fiberglass cleaner or auto cleaner, or even a kitchen abrasive pad.

5.   Scrub with a soft brush or fine kitchen abrasive pad, then rinse off with plenty of water.

6.   Scrub on with a soft kitchen abrasive pad, then wipe off with a clean coarse rag.

7.   The Plexiglas doors were sanded with plastic kitchen abrasive pads to give them a soft translucence.

8.   Then start scrubbing with a scrub brush or a kitchen abrasive pad wet with the same cleaner.

9.   Then use a wire brush to scrub the rusty area, let dry and scrub with steel wool or an abrasive pad.

10.   Use a wire brush or abrasive pad to remove the rust, then prime and paint the area.