Plaster pools are the most common type of inground pools in the USA. There comes a point when the plaster starts to get old and pitted. This is due to age and the years of chemicals eating away the finish. Many times plaster degradation is due to improper water balance. Some problems with a plaster finish can be caused by the mix or the application of the plaster. In any case, plaster is a natural product, and is meant to degrade. Hard enough to be durable, but soft enough to remove a thin top layer by acid washing.