No matter how well you lay your porcelain tile, a poor floor tile grout job can ruin the whole project. If the grout is laid too deeply in the lines, it won't protect the tiles at the edges as it's designed to do, and dirt and debris can build up in the cracks. If it comes up too high out of the lines, it will chip and break. Doing it right requires attention to timing, since the grout has to sit before and after application. Make sure the tiles are well set in their adhesive and have dried at least 24 hours before grouting.
Preparing to Grout Tiles
Remove the plastic spacers from between the tiles, using the edge of the putty knife to pop them out. Use a hand vac to vacuum any debris or dust that might have fallen into the cracks.
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Pour about 2 inches of cold water into the bottom of a bucket, then add powdered grout until the water is covered. Stir with the grouting trowel until well mixed. Continue adding water and power, alternately, until the mixture is 4 or 5 inches deep and the grout consistency is similar to that of thick mud.
Allow the grout to sit or "slake" for about 10 minutes. Stir again with the grout trowel.
Applying Porcelain Tile Grout
Scoop up a trowel full of grout and, starting at one corner of the room, press it into the lines between the tiles. Working in roughly four-square-foot sections, proceed across the room, pressing and wiping the grout down into the lines. The grout should completely fill the lines. After working in the porcelain tile grout, use a clean trowel to do another swipe across the grout area to remove any excess grout from the tiles.
Removing Excess Grout
Allow the grout in each section to dry for about 30 minutes. Fill a second bucket with cold, clear water, soak a thick sponge in the water and wring it out. Wipe the sponge across the tiles in a circular motion, keeping the wide side of the sponge flat on the surface. Apply steady but gentle pressure.
The goal is to wipe up the dried grout residue and smooth out the grout between the lines without digging it out. Dip the sponge in water and wring out again, and repeat until the tiles are clean and the only visible grout is what is set in the crevices of the tile. Allow the new grout to dry overnight. Thoroughly clean the tile after it dries.