Acid washing uses a diluted acid solution to strip stains, mineral deposits, and surface buildup from concrete, pool plaster, or brick. The process works because the acid reacts with calcium-based materials, dissolving a thin layer of the surface along with whatever discoloration is embedded in it. It’s straightforward but unforgiving if you skip safety steps or leave the acid on too long. Here’s how to do it right for the most common applications.
How Acid Washing Actually Works
Most concrete, plaster, and mortar contain calcium carbonate. When hydrochloric acid (sold as muriatic acid at hardware stores) contacts that calcium carbonate, it breaks it down into calcium chloride, water, and carbon dioxide gas. That carbon dioxide is the fizzing and bubbling you’ll see on the surface. The reaction eats away a thin layer of the material, taking stains, algae, efflorescence, and grime with it.
The bubbling is also your progress indicator. When it slows noticeably, the acid has finished reacting with the available surface material and needs to be rinsed off. Letting acid sit beyond that point risks etching too deep or causing permanent discoloration.
Safety Gear You Need Before Starting
Muriatic acid releases hydrogen chloride vapor that can damage your lungs quickly. Even bystanders walking through the fumes can start coughing after a single breath, and prolonged exposure without protection can cause lasting respiratory problems. At minimum, you need:
- Vapor-proof goggles (not just safety glasses)
- Rubber or chemical-resistant gloves
- A respirator rated for acid gases (not a dust mask)
- Impervious clothing that covers your arms and legs
- Rubber boots if you’re working on floors or in a drained pool
Work outdoors or in a space with strong airflow. If you’re working in an enclosed area, set up fans to push fumes away from you. Keep other people and pets well clear of the work zone.
How to Acid Wash Concrete
Acid washing concrete is most commonly done before applying a sealer, epoxy coating, or paint. The acid etches the surface just enough to give coatings something to grip. It also removes the smooth, glassy layer that forms on finished concrete and prevents coatings from bonding properly.
Preparation
Start by removing all dirt, dust, grease, oil, wax, and any other contaminants from the surface. Scrub with stiff-bristle brushes by hand or with a floor machine. Degreasers may be needed for oil spots. Any contamination left behind will block the acid from reaching the concrete evenly, leaving you with a patchy etch.
Once the surface is clean, wet the entire concrete surface with fresh water. You want it damp but without puddles. Pre-wetting prevents the acid from absorbing too quickly into dry concrete, which can cause uneven etching and burn marks.
Mixing and Applying
For concrete etching, a common starting ratio is one part muriatic acid to ten parts water for light cleaning, or up to one part acid to four parts water for heavier etching. Always add acid to water, never water to acid. Pouring water into concentrated acid can cause a violent, spattering reaction.
Apply the solution evenly across the wet surface using a plastic watering can or acid-resistant sprayer. You’ll see immediate fizzing. Let it work for 5 to 10 minutes, but never let the acid dry on the surface. When the bubbling slows significantly, scrub the surface with stiff-bristle brooms while flushing with plenty of clean water. This scrubbing step is critical because it removes the powdery residue that settles into the pores of the concrete.
Rinsing and Testing
Rinse the surface thoroughly, and then rinse it again. You may need to repeat the rinse more than once to remove all residue. After the surface dries, check for any remaining powdery film and brush it off before applying any coating.
Test the surface pH using pH strips. You’re looking for a reading of at least 6.3, ideally between 6.5 and 7.0. If the pH is still acidic, rinse again. A surface that’s too acidic will prevent coatings from curing properly. The finished texture should feel like medium-grit sandpaper, roughly comparable to 80 to 120 grit.
How to Acid Wash a Pool
Pool acid washing removes algae stains, mineral scale, and discoloration from plaster pool finishes. It’s typically done every five to seven years, or whenever staining can’t be resolved with normal chemical treatment.
Drain the pool completely and remove any debris from the bottom. Working from the top of the pool walls down toward the deep end prevents dirty runoff from contaminating areas you’ve already cleaned. Hose down the walls with water before applying acid, just as with concrete.
Mix equal parts muriatic acid and water in a plastic watering can with a long spout. Pour the solution onto the wall in a controlled strip and let it work. If the 50/50 mixture isn’t cutting through the stains, you can increase the acid concentration slightly, extend the contact time, or scrub harder with a stiff brush. Try to use as little acid as possible to avoid damaging the plaster. Rinse each section thoroughly before the acid dries, then move on to the next strip.
Collect all the acid rinse water at the deep end. This wastewater needs to be neutralized before disposal.
How to Acid Wash Brick
Brick acid washing is commonly done to remove efflorescence, those white salt deposits that leach out of masonry and mortar. It’s also used to clean excess mortar smears after new brickwork is completed. The best time to tackle efflorescence is during warm, dry weather, which helps prevent the salts from returning quickly.
Start with dry brushing and pressure washing. These methods handle many cases on their own. Chemical cleaning with an acid solution is the next step only when mechanical methods fall short on tough deposits.
One important caution: full-strength muriatic acid is generally not recommended for brick and mortar. It can stain or burn the masonry, etch mortar joints, and cause premature deterioration by opening the surface to water penetration. Specialized masonry cleaners formulated with milder acids are a safer choice. If you do use muriatic acid, dilute it heavily (typically 1 part acid to 10 or more parts water), pre-wet the brick, keep contact time to 3 to 8 minutes, and rinse thoroughly.
Safer Alternatives to Muriatic Acid
Sulfamic acid is a popular alternative for homeowners. It produces far less toxic vapor than muriatic acid, has lower overall toxicity, and won’t produce chlorine gas if accidentally mixed with bleach (a dangerous reaction that muriatic acid causes). It’s effective for removing rust, limescale, and light mineral stains from concrete and ceramics. The tradeoff is that it’s not as aggressive as muriatic acid, so heavy staining or deep etching may require more passes or additional scrubbing.
Phosphoric acid is another option commonly found in commercial concrete cleaners. It’s less volatile and easier to handle, though also less powerful on severe staining. For most residential cleaning jobs that don’t require deep surface etching, either of these alternatives will get the job done with significantly less risk.
Neutralizing and Disposing of Acid Waste
You cannot pour acid wash runoff down a storm drain or onto soil. The wastewater is corrosive and regulated as hazardous waste under federal environmental rules. It must be neutralized before disposal.
Baking soda (sodium bicarbonate) is the most accessible neutralizer. For concentrated muriatic acid, you need roughly 5.5 pounds of baking soda per gallon of acid. In practice, your acid is already diluted from the wash process, so you’ll need less. Sprinkle baking soda into the collected wastewater gradually, stirring as you go. It will fizz vigorously as it reacts. Keep adding until the fizzing stops and pH strips confirm the liquid is between 6.5 and 7.0.
Soda ash (sodium carbonate) works even more efficiently, requiring about 1.45 pounds per gallon of concentrated acid. It’s commonly available as a pool chemical.
Once neutralized, check with your local municipality about disposal. Many areas allow neutralized wash water to be discharged into sanitary sewer systems, but rules vary. Some require documentation or testing. Never discharge into waterways, storm drains, or onto the ground without confirming local regulations.
Avoiding Common Mistakes
The most frequent errors in acid washing all lead to the same outcome: permanent surface damage that’s expensive to repair.
Skipping the pre-wet step lets acid absorb unevenly, creating blotchy light and dark patches on concrete or plaster. Letting acid dry on the surface causes burns that no amount of rinsing will fix. Using acid that’s too concentrated when a weaker solution would work strips more material than necessary, shortening the lifespan of pool plaster or exposing aggregate in concrete.
Applying acid to a dirty surface wastes the reaction on grease and debris instead of the material underneath, leaving you with poor results and the temptation to apply more acid. And failing to scrub during the rinse phase leaves a chalky residue in the concrete pores that acts as a barrier, which defeats the purpose if you’re etching for a coating. Clean first, wet the surface, use the weakest effective concentration, time your contact carefully, and rinse aggressively. That sequence, done patiently, produces better results than a stronger acid applied carelessly.