We had a contractor install a flagstone patio. Apparently he didn't allow it to dry prior to sealing. The result is that it developed a white residue around the edges of stones. He has tried to fix it by scraping, removing the sealer, pressure cleaning, but it has not worked. When it rains the stone looks great. When it dries out the white residue reappears.
What can be done to bring back the beautiful natural colors?