We just finished a ceramic tile project on our back porch. We put in the grout, sponged down the grout lines and tile, and let it dry. When I went to wipe off the tile with a dry towel, there is a dull, dry film of grout that will not come off in some places. Apparently we did not sponge it off well enough.
Any suggestions on how to get this film off without damaging the tile?