We have a gunnite pool with Cool Blue Diamondbrite coating, 2 years old. We had an algae problem earlier this season, so we bought a couple different "black algae" stainless steel pool brushes and went after the algae.

In many of the places where we brushed with the metal brushes, we now have gray colored stains on the Diamondbrite. We read somewhere that this may be a metal stain due to manganese that is part of the stainless steel (?).

Has anyone else encountered this problem? Is there a known solution?

Thank you very much!