Recently, I had a tile guy installed black slates in my whole back yard. It was too late when I found out he was mixing concret on the existing ( already sealed) part of the patio. The patio was turned into light grey colour. A tile store manager in Santa monica asked us to use very strong acid to remove the residue of the concret and we did it and then we put wet look sealer on top of the slates. Now the grey is gone and we have about 10 huge white spots on the patio. How do I remove the white spots resulting from strong acid on the slates? I am desperately seeking help to rescue the $20,000 patio.