Remove grout haze with water and cheesecloth.
Cleaning grout residue from slate tile.
How to remove grout haze from a slate floor 1.
Since nothing you tried removed the grout haze i d guess that the slate probably wasn t properly sealed before the grout was applied.
Wring thoroughly as too much water can damage.
Protect your hands with rubber gloves and dip your cheesecloth or terry cloth in water.
Pick up the dirty water with a clean grout sponge.
Use another clean sponge to rinse the scrubbed slate tiles with.
Work on three or four.
Mix the grout haze remover put on nitrile chemical resistant gloves.
Instructions wait for the grout to cure wait at least 10 days for newly installed grout to cure before using grout haze remover.