OK. You've followed the manufacturer's direction, you got your grout on. You're very proud of yourself. But wait. Suddenly you see that you got grout on your tile and it's leaving a mark. Drat! The fact is, when you grout porous tiles like slate, marble or terra cotta any grout left on the tile can cause it to look foggy and dull. Unfortunately, there's not much you can do after the job is done and your floor is stained. You should seal the tile before you install it with either a top-coat or a pre-sealer. This will keep the grout from penetrating in the pores of the tile.