Thinset is a cement-base mortar used in tile applications. Amateur installers and even some experienced installers make the mistake of not wiping out excess thinset from the grout joints before it dries. The thinset then hardens and becomes a chore to remove. If you do not remove the thinset before the grouting, the thinset can show through the grout joints where it dried higher than the actual grout level.
During installation, use a damp sponge to wipe off excess thinset as you are setting it. For excess thinset in the grout joints, use a pencil, margin trowel or something that fits between the tiles without causing them to shift and remove the thinset. Wipe the tile one final time to remove any thinset on the surface sides of the grout joint.
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Remove dried thinset before grouting. Take a razor knife and gently scrape any dried thinset from the sides of the grout joint. You do not have to remove the dried thinset all the way to the cement slab. Just enough so that the thinset does show once the grout is applied. Use a shop vac to remove loose debris that might have fallen in the joint. Use a damp sponge to remove any discoloration on the tile surface or edge caused by the thinset.
Remove thinset that shows up during grouting before the grout dries. Use a razor knife and/or margin trowel in combination with a sponge to dig out the thinset and to wipe the edge. Remove the thinset from the work area to keep it from contaminating another grout joint. After removing the thinset, continue grouting the tile.
Examine the cured grout joints for exposed thinset. If any thinset is visible, you will have to dig through the grout to remove the thinset in applicable areas. Use the razor knife and/or corner of the margin trowel to dig it out. After removing the thinset, vacuum the grout joints with a shop vac to remove any loose debris from the grout joint. Wipe the areas with a damp sponge to clean the tile. Apply more grout to the applicable areas.