Grout is the mortar-like substance used to fill the joints between tiles. Once installed, grout requires very little maintenance. However, because it's porous, if the grout was not sealed, it may absorb stains. In addition, the mason may not have smoothed all the grout lines and you may find a rough spot. With a few basic sanding methods, you can smooth out a grout joint or remove a stain, but take care not to scratch the adjacent tiles during the process.
Fold a piece of medium-grit sandpaper to form a narrow sanding edge, then carefully sand back and forth in the grout joint without touching the tiles on either side.
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Sand with an angled foam sanding pad, using the thin edge to rub gently back and forth along the grout line. This alternate sanding method works well if you have many grout joints to sand, since the foam pad is large and easy to hold.
Roll a piece of sandpaper around a pencil or a dowel that's smaller in diameter than the width of your grout joint, then rub the "sanding stick" back and forth in the grout line. The stick sanding method gives grout joints a uniform, "rounded out" look.