Most concrete swimming pools are covered with a layer of plaster. Sometimes this plaster is set with tile, and sometimes it is covered with a waterproof layer to avoid leaks and deterioration. The concrete itself cannot be effectively waterproofed for a swimming pool, so the plaster layer is what keeps the water inside the pool. This layer of plaster is very important, but can develop problems or wear away over time.
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Scale is a common problem caused by hard water that contains too many minerals. A chemical reaction between the water and the minerals bonds these particles, often calcium or magnesium, to the plaster. This causes patches, known as scale, to appear on the plaster. While you cannot easily remove scale, you can soften water before using it in the pool, and replastering can solve many scale problems.
Etching is related to scaling, but is more connected to the condition of the water itself. Acidic water can cause pockmarks or rough areas in the plaster. Sometimes a discoloration will occur and the plaster will turn beige. This is not related to algae but instead points to low pH qualities of the water, which slowly eat away at the plaster itself. Materials in the concrete or plaster itself can also cause the acidic condition.
Plaster can also develop tiny cracks in the surface, known as crazing or checking. These are small, difficult to see and are not uncommon. They are most often caused by certain conditions when the plaster is first curing, such as extreme temperatures. They can also grow over time but do not endanger the structure of the pool itself. Since the cracks are only surface deep, they can usually be patched or acid washed to remove them.
This occurs when the plaster loses its adhesive hold onto the surface to which it has been attached. This is rare when the plaster bonds to concrete or gunite. It occurs most often in replastering situations when plaster is attached to another plaster or a primer surface that does not offer the best hold. You can solve this by removing all the plaster layers and patching the spot, although some structural problems might linger.
Pool conditions can eventually wear away the layer of plaster over time. You can usually see this because the plaster will develop darker, grayish areas. This is not a real discoloration, but rather the gunite or concrete showing through behind the plaster. When this happens, it is time to replaster completely.