Plaster must be in good order before you can repaint an existing pool. Pros recommend that you prepare the plaster surface with a wash using muriatic acids, then rinse it with TSP (trisodiumphosphate), and finally water. Note: Always use TSP (trisodium phosphate) and muriatic acids with extreme caution. It is important that the old paint be compatible with the new one. Rubberized latex paint, for example, won't stick to epoxy.
Concrete pools are plastered at the time of construction in the United States. Different pool coatings can be used in different areas of the United States when the original plaster becomes damaged or stained from years of acid cleaning and poor water chemistry. The pool coatings are then applied over the plaster. The plaster must be in good condition. If it has spalled from the concrete shell below, it will need to be repaired.
The most common in-ground pools are concrete shells that have been finished with a type of Portland cement-based material, known in the pool trade as “plaster,” that seals and protects the shell. In many instances, the plaster is coated with paint, epoxy, or even fiberglass. The plaster can also serve as a substrate for tile.