The smooth finish on gunite pools. Usually white, colored plaster has become increasingly more popular, recently. Plaster is a hand applied, hand troweled product. Plaster is the combination of cement, aggregate, and admixtures that form the decorative, aesthetic, maintainable shell of the pool. The functional purpose of plaster is to be watertight and smooth. The plaster has no structural strength. A SMOOTH FINISH is defined as having a texture no rougher than very fine sand paper (220 grit). A smooth finish allows the pool to be more easily maintained and serviced. A COLORED PLASTER can help to reduce the number of visible stains in pools with trees (leaves will cause plaster stains) or with stonework at poolside (mineral stains). A method of removing plaster to roughen the finish before replastering is called ACID WASHING. Some people who like to acid wash the pool to clean up the stains are just preparing the pool, for a replaster, a little early (although, most plaster crews now seem to prefer a bonding primer coat). Adding a 'waterproofing' agent to the plaster, an 'armor coat', can greatly enhance the life of the finish. Similar (although not as smooth) finishes are 'Diamond Brite' (smooth) and 'Pebbletech' (very thick & rough, no tile), both use quartz instead of marble, and they both seem to last far longer than a standard marble based finish.