How Do You Acid Wash a Pool Without Damaging Plaster?

The Short Version

• Acid washing removes a thin, microscopic layer of plaster to expose clean, bright material underneath, using a balanced mix of water and muriatic acid (a strong liquid acid).
• You can safely use this method only on plaster, gunite, or pebble aggregate finishes; applying acid to a vinyl liner or fiberglass shell will ruin the material.
• The acid mix must sit on the wall for no more than 30 to 60 seconds before you scrub it and flood the plaster with clean rinse water to prevent etching (eating away the surface layer).
• Before pumping the pool runoff into a drain, you must neutralize the puddle at the bottom with soda ash (acid neutralizer powder) to protect your plumbing and the environment.

Which Pool Surfaces Can Be Acid Washed

Acid washing is an aggressive deep cleaning method designed strictly for hard masonry finishes. It works on traditional white plaster, dark plaster, gunite, and pebble aggregate finishes such as Pebble Tec. Over several seasons, minerals like copper and iron bind to plaster pores, while stubborn algae spores take root deep in microscopic crevices. Standard scrubbing and high chlorine levels cannot always lift these deep discolorations. An acid wash strips off the top sliver of the plaster surface, taking the embedded stains along with it and revealing clean plaster beneath.

You must never acid wash a vinyl liner pool or a fiberglass pool. Vinyl liners are thin sheets of polyvinyl chloride plastic. Muriatic acid pulls the plasticizers out of the vinyl, which makes the liner brittle, causes ugly wrinkles, fades the print pattern, and eventually burns holes right through the floor or walls. Fiberglass pools use an outer gel-coat resin that creates a smooth, glossy shield. Strong acid strips this smooth gel coat, leaving the surface rough, dull, and prone to blister leaks that require expensive fiberglass resurfacing.

Even on plaster, you cannot repeat this process indefinitely. Traditional pool plaster is only about three-eighths of an inch to one-half of an inch thick. Because each treatment dissolves a small layer of material, a typical plaster finish can handle only three to five washes in its entire lifespan. If your plaster is already thin, worn rough, or flaking off, an acid wash will eat right down to the gray concrete gunite underneath. When gray patches are already showing through the white plaster, the pool needs new plaster, not an acid treatment.

You also need to evaluate groundwater before you commit to draining your pool. When looking at how deep are inground pools in standard backyards, remember that millions of pounds of water weight hold the structure down. If your yard has a high water table from recent heavy rain, the pressure beneath an empty pool can push the entire concrete bowl up out of the earth. Always remove the hydrostatic relief plug (a floor plug that stops underground water from lifting the pool) found inside the main drain before you begin working on an empty pool.

Essential Safety Gear and Tools

Muriatic acid is dangerous to handle, especially inside an empty pool bowl. The liquid produces heavy, pungent fumes that settle at the bottom of the deep end rather than blowing away on the breeze. Breathing these fumes causes immediate coughing and lung irritation, while splashes cause serious chemical burns. Gather all protective gear before you open a single chemical jug.

• Dual-cartridge respirator: Wear a respirator fitted with approved acid gas filter cartridges. A paper dust mask or cloth cover offers zero protection against acid fumes.
• Chemical splash goggles: Use fully sealed goggles with indirect venting so liquid cannot splash into your eyes from the side.
• Heavy rubber boots: Wear tall waterproof boots with non-slip soles. Acid will soak through regular sneakers and burn your feet.
• Acid-resistant gloves: Choose thick rubber or neoprene gloves that extend past your wrists to protect your arms from accidental drips.
• Plastic watering can: Use a standard two-gallon plastic garden watering can with a sprinkler head. Never use metal cans or sprayers, because acid corrodes metal instantly.
• Two garden hoses: Keep one hose running continuously to keep dry walls wet, and use the second hose with an adjustable spray nozzle for fast rinsing.
• Acid brush: Buy a stiff-bristled pool scrub brush mounted on a standard extension pole. Never use a stainless steel wire brush, which scratches plaster and leaves metal bits that rust.
• Soda ash: Keep at least 20 to 30 pounds of soda ash (acid neutralizer powder) on hand to treat the waste puddle on the pool floor.
• Submersible pump and discharge hose: You need an electric sump pump to remove the neutralized liquid from the bottom drain area.

Step-by-Step Acid Washing Technique

Acid washing moves quickly once you begin. You need to work systematically, treating one small vertical strip of the wall at a time so the acid never sits long enough to eat deep pits into the plaster.

1. Drain and clear the pool: Pump out the pool water completely using your submersible pump. Take out the hydrostatic relief plug at the bottom drain to prevent ground pressure issues. Use a garden hose to rinse away loose leaves, dirt, and heavy algae slicks so the acid can make direct contact with the plaster.
2. Mix the acid solution: Fill your plastic watering can halfway with fresh water first, then slowly pour in the muriatic acid to create a 1:1 mixture. Always pour acid into water, never water into acid. Adding water to concentrated acid can trigger a sudden heat reaction that spits burning liquid into your face. For lighter staining, you can start with a milder mixture of one part acid to two parts water.
3. Wet the plaster thoroughly: Spray the section of wall you plan to clean using clean water from your garden hose. The plaster must be completely wet before the chemical touches it. If you apply acid to dry plaster, the chemical absorbs too deeply into specific dry spots, causing uneven bleaching and permanent streaks.
4. Apply the mixture from top to bottom: Stand safely above or beside the section and pour the mixture along the top of the wall just under the tile line. Work across a section that is three to four feet wide. The liquid will foam and bubble as it reacts with the mineral surface, releasing white vapor.
5. Scrub the surface lightly: Use your pole-mounted brush to spread the bubbling liquid across the stained area. Scrub firmly over mineral rings or dark algae marks to help the chemical lift the stain. Keep the mixture moving so it coats the plaster evenly.
6. Rinse within 30 to 60 seconds: Never leave the acid mixture on the wall for more than one minute. Grab your rinse hose and flood the entire section with a heavy spray of clean water, working from the top down. Wash every trace of acid down toward the bottom drain to stop the chemical reaction.
7. Move around the pool: Repeat this process one panel at a time, moving from the shallow end toward the deep end. Complete all the walls first. Once the walls are clean, wash the pool floor in sections, pushing the rinse water toward the main drain.

Neutralizing and Pumping Out the Slurry

The rinse water that gathers in the deep end bowl is called acid slurry. It is a milky, highly acidic puddle filled with dissolved plaster minerals, dirt, and spent acid. You cannot pump this slurry straight into a street drain, a storm sewer, or your grass. Pumping raw acid liquid violates local environmental codes, kills your landscaping, and ruins the rubber seals inside your submersible utility pump.

To make the waste safe, broadcast soda ash (acid neutralizer powder) over the entire deep end puddle. A reliable rule of thumb is to add roughly two pounds of soda ash for every gallon of muriatic acid you poured onto the pool walls. Use your pool brush on its pole to stir the bottom puddle thoroughly. The slurry will bubble and fizz vigorously as the soda ash cancels out the acidity, producing safe carbon dioxide gas and salt water.

Continue stirring until all bubbling stops. Drop a pH test strip into the puddle to verify the reading. The liquid should register between 7.0 and 8.0 on your scale, which matches how carefully you monitor water balance when deciding how often to check pool chemicals in an operating pool. Once the liquid is neutral, turn on your submersible pump and send the water to an approved sewer cleanout port. Use a garden hose to rinse the remaining chalky white paste toward the drain bowl, shovel out any heavy grit, and pump away the final rinse.

Critical Mistakes That Ruin Pool Plaster

Acid washing is an effective tool, but small errors in timing or technique can cause severe plaster damage that only a complete replastering job can fix. Watch out for these common errors during the project:

• Letting acid sit too long: If you leave acid on the plaster for two or three minutes, it causes severe etching (eating away the surface layer). This turns smooth plaster into rough, sandpaper-like texture that scrapes bare feet, snags swimsuits, and traps fresh algae faster than before.
• Working under direct midday sun: Hot sunshine dries the acid onto the plaster before you have time to scrub and rinse it off. This creates yellow burn marks and blotchy stains. Plan your project for early morning hours or a cool, cloudy day.
• Applying chemical to dry plaster: Pouring acid onto dry spots leaves dark drip lines that run from the tile down to the floor. Plaster must stay soaked with hose water right up until the moment you apply the mix.
• Pouring water into acid jugs: Always add acid to water in your watering can. Reversing the order causes splash-back that can burn your skin and eyes.
• Leaving the pool empty for days: Once you finish washing and neutralizing, start refilling the pool immediately. Plaster left exposed to hot air and sun shrinks, blisters, and forms tiny spiderweb cracks known as crazing. Once your pool is full of fresh water, you can start balancing the chemistry to see how to get your pool water sparkling blue again.

FAQs

How often can you safely acid wash a pool?

You should acid wash a plaster pool no more than once every five to seven years. Because each treatment strips off a small layer of plaster, doing it more frequently will wear the finish down to the rough concrete base. Reserve acid washing for severe algae takeovers or heavy mineral staining that normal pool chemicals cannot clear.

Can you acid wash a painted swimming pool?

No, you should never acid wash a painted pool surface. Muriatic acid strips and softens swimming pool paint, causing it to bubble, blister, and peel away in large sheets. Painted pools require specialized rubber-base cleaners or gentle pressure washing rather than strong mineral acids.

What should you do if muriatic acid splashes on your skin?

Flush the affected skin immediately with large amounts of clean, cool water for at least fifteen minutes. Keep a running garden hose right next to your work station at all times so you do not lose time looking for a water source. If burning, redness, or blistering continues after thorough rinsing, seek medical attention right away.

How long after acid washing can you swim?

You cannot swim until the pool is completely refilled and your circulation system has run long enough to balance the water chemistry. Refilling takes one to two days depending on water pressure, and stabilizing the pH, total alkalinity, and sanitizer levels takes another 24 to 48 hours. Most pool owners can safely swim three to four days after finishing the acid wash.