The most commonly performed surface revitalization technique is acid washing. That’s because acid washing can do wonders to remove stains and other discolorations.
That said, acid washing is not an answer for every problem. For example, if the plaster is approaching the end of its life expectancy, it may be time to re-plaster rather than further weaken the surface with acid.
Furthermore, it should not be used on pools where the plaster has lifted or blistered, a condition called spalling or delamination. This is considered a cementitious failure, and aggressive water can further deteriorate the surface. Such pools may ultimately require resurfacing.
Acid washing is also no solution for calcium nodules, for which there is no chemical treatment, and which will also require resurfacing.
Acid washing is appropriate for surface stain removal from metals, scale formation, and algae. Acid washing is also done on newly set aggregate surfaces, such as pebble or bead-embedded plaster because it helps to expose some of the aggregate.
In the service industry, acid washing is a fairly commonly offered service. Last year’s Service Industry News survey revealed that in both California and Florida for example, of those responding to the survey, about 50 percent included acid washing among their offered services.
Acid washing begins with draining the pool. As always, when draining a pool, great care must be taken to ensure that hydrostatic pressure will not result in the pool popping. To protect the pool, open any hydrostatic relief valves or plugs.
It is recommended to use a separate pump that is not part of the circulation system to drain residential pools. Take care to monitor the discharge water to protect lawns and shrubs and to be sure neighboring properties aren’t flooded. The water must be drained far enough away so as to avoid washouts or hydrostatic problems.
When the shallow end is revealed, start the procedures for the acid wash, beginning by cleaning dirt and oils away with a low sudsing detergent.
Acid washing involves using the extremely caustic chemical, hydrochloric acid, commonly called muriatic acid in the industry.
Therefore it’s necessary to protect against injury by wearing the appropriate protective gear, which includes a gas mask respirator, rubber gloves, rubber boots, a suit, and goggles. The fumes are toxic and the gas mask is strongly advised.
Muriatic acid is commonly sold in a 20° Baume strength. This is a 32% dilution of hydrochloric acid. Further dilutions of 32% hydrochloric acid appropriate for acid washing or etching are described as follows: For an acid wash, a 3 to 4% hydrochloric acid solution is recommended. Always add acid to water, never the reverse, as an explosion may occur. To obtain 4% dilution strength, mix 1 gallon of 32% HCl with 7 gallons of water.
For an acid etch, a 10% hydrochloric acid solution is recommended. Always add acid to water, never the reverse, as an explosion may occur. To obtain 10% dilution strength, mix 1 gallon of 32% HCl with 2.2 gallons of water.
Acid etching is similar to acid washing except that a higher concentration of acid is used. Etching is appropriate for the surface preparation involved in painting or replastering.
These acidity strengths are general guidelines, and specific circumstances will call for different acid strengths. It may be necessary to adjust the dilution when treating a painted pool or for various densities in the masonry finish.
Image credit: Alan Smith Pool Plastering It’s a good idea to mix the recommended dilution and then scale up or down as the situation calls for it.
To get started, wearing personal protective equipment, prepare the solution, and then begin applying the solution with a plastic sprayer. Work in sections. Don’t attempt to apply the acid to the entire pool at once. Badly stained areas should be scrubbed with a non-metal brush and hose rinsed when the foaming has stopped or if the area is sufficiently cleaned.
Don’t allow acid to dry on the plaster because it will damage the surface.
Not all stains can be removed with an acid wash, although some stains may require multiple applications to come out. Experts agree that it is better to apply multiple doses of a weaker solution than accidentally using too strong a solution.
When the wash is complete, a soda ash solution is used to neutralize the active acid on the plaster surfaces. It also neutralizes the waste water that has accumulated to the EPAacceptable pH level of 6.0 to 8.0. It’s preferable to neutralize to a pH of 7.0.
Before refilling the pool, rinse all surfaces with fresh water, and dispose of the waste water as per local regulations.
After acid treatment is complete, be sure to flush out the plumbing to prevent corrosion.
The pool may then be refilled by placing the hose carefully in the deep end, unless further treatment, such as paint ing or torching, is planned.