Remove excess thin set quickly
Thin set is made to hold tiles firmly to the wall, so when it grabs hold of something and dries, it can be difficult to remove. The best solution is to keep tubs and other surfaces covered with a canvas drop cloth or plastic sheeting when applying thin set. Wipe excess thin set as quickly as possible before it dries.
Wipe excess thin set with a damp sponge before it dries. Rinse sponge often.
Scrape drops of dried thin set with a plastic trowel or spatula, taking care not to scratch the tub.
Mix equal parts of white vinegar and water in a spray bottle. Soak areas of excess thin set with this solution. Wait at least 10 minutes and wipe with a damp sponge.