Service professionals generally use muriatic acid for acid washing. This acid is usually packaged in a 20° Baume strength, or a 32% dilution of hydrochloric acid. For acid washing, experts recommend a weak to medium strength concentration of from 3 to 10%. The 10% solution may etch a bit of the plaster, however. To obtain a 4% solution, mix 1 gallon of 32% muriatic acid with 4 gallons of water. For a 10% solution, mix 2.5 gallons of 32% muriatic acid with 2.5 gallons of water. Remember to always add acid to water and never the reverse.

Wearing personal protection gear, work with a partner in sections, taking all precautions to avoid splashing. Beginning in one section, one partner should apply the solution to the surface with a plastic garden can or sprayer, while the other partner scrubs the area with a non-metal brush. The partners should work quickly together, with one scrubbing vigorously while the other follows with the hose to prevent streaking.

Soda ash, sodium carbonate, should be poured where areas of waste water accumulation to neutralize the water.

The acid should not be allowed to dry on the pool surface because it will damage the surface. Each section should be finished before moving on to the next section. Stubborn stains will sometimes require multiple applications, but take care not to over apply the acid, and be aware that not all stains can be removed.

A solution of soda ash, mixed in water, is then used to wash the entire pool shell, which will neutralize any acid remaining on the surface and raise the pH of the waste water to acceptable levels. Finally, rinse all of the surfaces with plenty of water.

A sump pump should be placed at the bottom of the deep end to remove the standing water there, which will leave marks on the pool floor if allowed to stay there.

Test the waste water left in the pool to determine its pH. If the pH is less than 2.0, the solution should be removed to plastic drums and disposed of at a hazardous waste facility. If the pH is less than 5.4, soda ash can be added to neutralize to an acceptable level of between 6 and 8. In addition to these general instructions, there are a few extra facts and tips that may help to improve the job:

• Acid additives are available that can help to improve the acid’s performance. These are thickening agents that allow hold the acid to the wall a little longer for better penetration. This can help with streaking problems as well. They also promise to cut down on fumes to make the job less unpleasant.

• Commercial quality brushes are available with curves to help get into edges and steps.

• It takes 40 pounds of soda ash to neutralize 5 gallons of muriatic acid.

• If the pool has an exceptional calcium content, experts recommend giving the pool an acid soak before the acid wash. Add one gallon of acid per 1,000 gallons of water about 10 days prior to the planned acid wash. Take care not to use the equipment during this period because the low pH involved will damage it. Also, plug skimmer and return lines if there is copper plumbing.

• Limit the amount of time that the pool is empty. Drain the pool the day before the work is to be done.

Do not allow pool to remain empty for longer than 5 days.

• After draining, review the quality of the plaster with the customer, pointing out any defects or new problems. Provide them with reasonable expectations.

• Drain pool water to a legal and appropriate location.

• Don’t work alone. It is not only unsafe, but a two-person team has much better control over the acid, brushes, and hose.

• When the wash is complete, the team should review the surface to determine if another wash or a higher concentration wash is in order.

•Any roughened areas or remaining rust stains can be smoothed or removed by sanding with wet-dry paper.

• After the acid wash procedure is finished, refill the pool as quickly as possible. As with newly plastered pools, do not stop filling until the water reaches halfway up the skimmer. Stopping water flow can cause a ring around the plaster.

• Ensure that there is a responsible party present to stop the water flow when the filling is complete. Fill water left unattended can flood homes and cause other calamities.