Step 6: Grout the Joints

The next day, wash away thinset residue with muriatic acid, and, to protect the stone from grout stains, apply a sealer. When that's dry, mix up a sanded grout and push it diagonally into the joints with a grout float. Wait 10 minutes, then wipe off grout residue from the stone with a damp sponge, rinsing it often. Don't disturb the joints. The next day, rub off any haze with cheesecloth and wipe on a final sealer coat.
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    Tools List

    • utility knife with hook blade
      Utility knife
    • mixing drill
      Corded drill with mixing paddle to mix thinset and grout
    • small plastic bucket
      Buckets for thinset, grout, and rinse water
    • notched trowel
      ¼-inch notched trowel
    • grout float
      Grout float
    • margin trowel
      Margin trowel to spread thinset on back of stone
    • plastic putty knife
      Putty knife
    • sponge
    • Cheesecloth

    Shopping List

    Uncoupling membrane
    Thinset, modified and unmodified
    Muriatic acid
    Slate tiles
    Penetrating stone sealer
    Sanded grout