Black Dot Stains

[thabeezer]

Not sure if this is the correct place for this question so I apologize in advance if it's not.

I have flagstone coping that has little black dots that look like mold. I tried to use bleach solution and SS brush but they would not budge.

Any idea what they are and how to get rid of them.

 

[Melt In The Sun]

Pictures needed!

 

[thabeezer]

Pics

 

[thabeezer]

 

[JamesW]

Put a few drops of straight bleach on one of the dots and let the bleach completely dry/evaporate before rinsing to see what that does.

 

[thabeezer]

Put a few drops of straight bleach on one of the dots and let the bleach completely dry/evaporate before rinsing to see what that does.

That just bleached the stone a little..

 

[JamesW]

I think that the bleach test probably rules out mold or lichens. It might be some type of metal oxide in the stone, perhaps manganese, iron, copper or cobalt. You could try some white vinegar on a test spot to see what happens.

You could also try ascorbic acid (vitamin C) or oxalic acid. Barkeeper's friend contains oxalic acid.

Oxalic acid

You could also try a sulfamic acid based cleaner.

Another possible solution is Super Iron Out Rust Remover, which contains sodium metabisulfite, and sodium hydrosulfite.

Note: Acids might damage the stone. Always try a test spot to be sure. Acids might weaken the stone and the damage might take a while to become evident, so there is some risk from using acids.

Another thing you could try is trisodium phosphate (TSP). Most TSP is a TSP substitute; you should make sure that the box actually lists trisodium phosphate on the label.