Natural stone tile is a lot more porous than it looks. It should be sealed with a liquid chemical sealant after installation to prevent it from absorbing moisture and stains. The grout between the tiles should also be sealed. Sealing is done in two parts--before and after the grouting--because grout will stain unsealed tiles. Install the tiles, seal the tile faces, grout the tiles (the grout won't stain the tiles if they're sealed), then apply a second layer of sealant over the whole tiled area, grout and all.
1
Lay your stone tiles with consistent spacing between the tiles (usually 1/8 inch) and allow them to set overnight.
2
Brush tile sealant onto the surface of the tiles with a paint brush, starting in the corner of the room furthest from the doorway. Brush it on in a thin, even layer, keeping it just on the face of the tiles, without letting it drip inside the spaces between the tiles. Let it dry for eight hours.
3
Grout the tile, using your grout float to spread it across the surface, pressing it into the spaces and squeezing it off the top. Work in sections of a few square feet, letting the grout sit in the spaces for 5 to 10 minutes, then wiping off the excess with a damp sponge. Let the grout cure for at least 72 hours.
4
Seal the whole floor (tile and grout), working from the far corner of the room. Let the sealant cure for two to three days before resuming use of the area.
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