Natural stone tiles make beautiful floors, walls, counter tops and shower enclosures. Properly sealed and grouted, the installation will last a lifetime with reasonable care and maintenance. Porous tile requires extra steps before, during and after grouting to protect the tile from staining.
Clean the tiled area thoroughly. Remove any mortar lumps left between the tiles and vacuum any dust. Wipe the tiles with a damp rag or sponge mop.
Seal the tile if this was not done before installation, following the manufacturer's instructions. Allow the tile sealant to dry completely (overnight) before applying grout.
Mix the grout according to instructions and fill the grout bag. Filling the bag more than half full will make it harder to control the grout flow.
Work in a 3 x 3 foot area. First dampen the tiles with a wet sponge, but not to the point where there is standing water on or between the tiles.
Fill the tile seams with grout from the grout bag, keeping the tiles clean. Apply just enough grout to the seams to slightly overfill them.
Dampen the grout float and work the grout into spaces between the tiles. Clean the tiles as you work with a damp sponge. If desired, smooth the joints by going over them lightly with a damp sponge, but not pressing into them.
Allow the grout to set up for two to four hours. With a clean, damp sponge, go back over the tile and remove any grout haze left behind. Do not dig into the grout lines, as this will damage them.
Wait two days for the grout to finish curing. Apply sealer in two coats over the grout lines.
Using the grout bag prevents a build-up of grout on the tiles, making cleaning up afterward easier and faster. If the tiles were not sealed, you won't worry so much the tile colour will leach into the tiles or that grout will be stuck in the pores of the stone. If the grout lines between tiles are small, less than 1/8 inch, add just a bit more water when mixing the grout. The added fluidity of the grout mix will help you fill the seams fully. For narrow grout lines, buy a cheap cake icing bag with tips. Using a narrow tip with the grout will help keep the grout in the lines and off the tile.
Always wear safety glasses when mixing and applying grout. Grout contains silica cement products which can scratch and damage your eyes. Always wear a dust mask when mixing grout to protect against inhalation of the silica dust in grout. Don't allow grout to sit on unsealed porous tile. The tile's pores may absorb the colour or grout, permanently altering the natural surface.