I walked into my son's house to find him and his friend frantically trying to wash the grout off the tub surround tiles before it completely dried. He's tiled several houses beautifully and mixed the grout exactly according to the instructions on the package. I helped them as much as possible, but we couldn't get all the grout off the tiles.
We're in Boise, Idaho. Weather is humid, low 50s with occasional rain during the day. the bath is in the basement. Substrate is green board.
Anyone know what might have happened?