Let’s be honest: Pool stains suck.
They’re unsightly and usually indicate a maintenance, chemical balance or underlying water issue. While the root causes should be treated, the stains themselves may need extra attention.
Acid washing is a pool cleaning process that uses a combination of water and hydrochloric acid, or muriatic acid, to remove stains, algae and other buildup from a pool’s surface when routine cleaning and scrubbing aren’t enough. The goal, however, isn’t just to remove the buildup from the surface — it’s to do it without damaging the plaster.
But acid washing isn’t as simple as an over-the-counter remedy. It’s an aggressive maintenance procedure that should only be done by a trained professional when necessary.
Alan Smith, founder of Alan Smith Pools in Orange, California, believes the industry has “dropped the ball” when educating on best practices for acid washes.
Smith has been plastering pools since 1974, is a founding member of the National Plasterers Council and chaired the research committee for the Environmental Engineering Department at California Polytechnic State University.
Smith emphasizes the importance of pool pros knowing the “why” behind what they’re doing because it shapes their understanding of “what” and “how.”
“I don’t think they quite understand what’s going on chemically between the material and the acid and the bicarb,” Smith says. “A lot of people know how to do things, but they don’t know why they do what they do.”
Smith says that educating professionals about what’s going on at the chemical and material levels is crucial for avoiding mistakes.
Chris Marcano, executive director of the National Plasterers Council, advises acid washing only be done by trained professionals who understand the balance between effective cleaning and preserving plaster longevity.
One frequent mistake that both Smith and Marcano have seen is pool pros using too strong an acid solution, which Marcano says can excessively etch beyond the surface issue and create rough, uneven plaster textures.
“Applying acid unevenly also leads to streaking or patchy results,” Marcano says. “Always prewet the plaster to prevent acid from penetrating too deeply.”
Pool pros should avoid applying acid under intense sunlight, as this accelerates evaporation and affects uniformity. Follow manufacturer dilution guidelines carefully, as overly concentrated solutions increase the risk of damage. Marcano says the entire pool surface should be rinsed in a bicarb and water solution before refilling.
Smith says his team takes a soft approach through an even-spray method, using a 35-gallon trash can, plastic sprayer and diluted solution to spot-treat if the entire pool doesn’t need it.
“[We do that] rather than the old-fashioned way of going around with watering cans with 100 or 50% acid, pouring it on and just kind of hitting that whole pool with a Mike Tyson punch,” Smith says. “You don’t have to do the whole pool.”
In addition to gloves, goggles and a respirator, it’s important to use acid-resistant footwear and keep a water hose nearby to quickly rinse any spills. Ventilation is a pool pro’s best friend and crucial to avoid inhaling fumes.
Marcano says to also keep a bucket of water and baking soda handy to neutralize any spills or accidental skin contact. Don’t forget to rinse tools with a baking soda solution thoroughly to prevent residual acid from corroding them. Smith says adding Dawn dish soap to the mixture is a simple way to weaken fumes and help the acid stick to the sides of the pool better.
Marcano and Smith agree that environmental safety is a major consideration when acid washing. “Always neutralize the acid solution with a neutralizing agent, like baking soda or soda ash and more water, before disposal to prevent harm to local waterways,” Marcano says. “Be mindful of any runoff, ensuring it does not flow into storm drains or nearby landscaping.”
Always neutralize the acid solution with a neutralizing agent, like baking soda or soda ash and more water, before disposal to prevent harm to local waterways. Be mindful of any runoff, ensuring it does not flow into storm drains or nearby landscaping.”
Chris Marcano, National Plasterers Council
Marcano points out that some regions may have strict regulations on acid disposal to protect groundwater and ecosystems and advises pool pros to research and comply with local ordinances.
Smith suggests researching other supplemental products and chemicals and points to products like Pool Wash that are environmentally friendly and good for a pool’s finish.
Marcano says moderate weather conditions are ideal for acid washing, as extreme heat causes acid to evaporate too quickly, while cold conditions slow its effectiveness.
“High humidity can make fumes more intense and challenging to dissipate, and rainy or windy weather is also not recommended as it can dilute the acid or blow fumes into surrounding areas,” Marcano says. “Mild, dry days provide the best conditions for safe and effective acid application.”
Equipment Marcano recommends includes a chemical-resistant plastic sprayer for applying acid evenly, acid-resistant brushes to prevent corrosion and a neutralizing agent to safely treat the acid before disposal. He says a submersible pump can help remove the acid solution from the pool floor, and it’s wise to have a backup hose for quick rinsing of both the surface and tools.
Marcano says investing in quality, acid-rated PPE is essential for safe handling. Look into alternatives to acid washing first, particularly in pools with soft water where minor stains are more easily removed without stripping the plaster.
“Acid washing is an aggressive maintenance procedure and should only be used when necessary,” Marcano says. “Regular maintenance, such as saturation-index-balanced water chemistry, weekly brushing and prompt stain treatment helps prevent stains from settling in the first place.”
