Pool plaster is the final coating applied to the shell of a concrete in-ground pool. Pool plaster adds a watertight seal and makes the surface of the pool smoother for contact with swimmers than the underlying rough concrete shell. It's also instrumental in creating a pleasing aesthetic appearance. There's nothing like a swimming pool to turn a basic backyard into the ultimate staycation destination. Of course, if you want to keep your swimming pool beautiful, clean, and healthy, you'll need to know how to care for a pool.