Last summer I cleaned and sealed some exterior stone for a customer. He called me after and said that there were spots on the stone on three steps (see photo) which I thought might have been caused by sealer drips. (I used a water based sealer). So I went back and tried to remove these mars first using alkaline and then acid.

Then, the winter came and I heard nothing from this guy until last week. Today he sent me these photos and is very upset that I ruined his stone. The stairs, are where the problem started. but there is damage to the walk way as well (which I did not seal, only cleaned.) Now he says he wants his money back because he needs to replace all the stone in his yard.

My first question is what did I do ?

My second question is what should I do ???