There are two commonly used acid washes , no drain and drain the pool. The no-drain acid wash is where the water remains in the pool and is prepared by adding acid to the water, about 12 gallons. The water becomes very caustic which loosens the stain on the plaster. By brushing the pool every day with the system off, the loose plaster comes off and settles to the bottom. Brushing the pool is necessary for good results. Use a stiff bristled brush over the entire pool. After a week or when the appearance of the pool is better, the adding of soda ash to the water is used to raise the ph to about 7.4, several pounds are necessary. The system can now be turned on and the residue vacuumed out, preferably by a porta-vac. This process works well where the draining of pools is mandated by the city and where time is not a factor. The draining acid wash is where the water is drained and the plaster is washed with an acid solution with about 50% water, also a little tile soap can be added to help it stay on the walls. Using an acid wash brush, the plaster is scrubbed until the desired appearance is achieved. A pump is used during the process to keep the pool empty of water and the acid solution. This technique takes a couple of days and the end results are better. The pool can now be filled with fresh water and conditioned. After the chemical balance is adjusted in the water, the clean pool can now be used. A clean looking pool is a welcome environment for the avid swimmer.