You make a beautiful mosaic, grout it, and then the next day there is white powdery streaks running down off of the grout. What a mess! What is this white stuff? How do I get rid of it?? The white stuff is called efflorescence. It happens most often when you apply grout in moist and cool conditions. In my case, I grouted between rains when it was foggy but dry. The fountain showed no signs of white streaks and was sent out on a truck. A few days later I went down to do finishing touches on it and saw this:
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