Pool plaster is made up of cement, sand and water. It is commonly troweled onto a concrete pool shell in 3 to 5 separate passes – the early passes to place the material and the later passes to create a smooth final finish. After plaster is troweled, excess water will bleed to the surface. Bleed water then evaporates from the surface. There are two common mistakes made during troweling. First, if troweling is completed when bleed water is present it will force water back into the plaster paste which causes excessively high water to cement ratio which weakens the finished surface. Second if troweling is completed late after the surface is too dry a crust will form with a wet paste underneath. This will create a weakened zone subsurface. This typically happens on dry, hot days with low humidity and wind. If this happens, the finished surface will look fine and even last awhile if the pool is full of water. However, when the pool is emptied, a 16th to an 8th inch layer of plaster will flake off or spall in small areas or spots. Pool plaster spalling is a rare occurrence but most often happens in areas that are challenging to apply plaster including step areas, main drains, and shallow areas. Often, the first reaction to pool plaster failure is to blame the pool water chemistry however improper installation is typically the cause of pool plaster spalling.