Acid washing, also known as acid etching, is the process of preparing the concrete surface before applying a sealer. You can also use a low concentration of acid to remove white mineral deposits (swelling) and heavy impurities. Acid washing is harmful to people, plants, and metal objects, especially if done indoors as the fumes will collect there.
Do not confuse this process with acid staining, which is the process for coloring concrete. Acid washing is not recommended before you do acid staining.
Step
Part 1 of 3: Getting Ready
Step 1. Remove grease and dirt
Brush or vacuum any adhering to the concrete. If there are oil stains, you can remove them with a concrete degreaser or alkaline detergent. Rinse with water until clean.
- If there is water on the surface, the acid wash will not be complete. This can be solved with a degreaser.
- You should not use a trisodium phosphate (TSP) cleaner. The residue left behind can react violently with the acid to produce harmful gases.
Step 2. Use the desired acid
Choose an etching or cleaning product based on your level of experience and the location of the acid wash:
- Sulfamic acid is the safest product to use, and is recommended for non-professionals.
- Phosphoric acid produces a small amount of smoke. You can use this product in rooms where stainless steel or other metals are susceptible to acids. This material can also be used to clean mineral deposits.
- Muriatic acid (hydrochloric acid) is the most dangerous and emits a strong fume. This product is only recommended for professionals who do it outdoors.
Step 3. Follow the safety instructions
The acid used in this process is a dangerous household cleaning product. Wear acid-resistant gloves, rubber boots, and vapor-resistant goggles. Wear a respirator with an acid filter to protect the lungs, and if necessary use a fan for better ventilation. Protect potentially acid-prone skin with clothing that covers the body, ideally one that has a face shield plus a coverall (project worker attire) or an apron (apron) made of PVC or butyl.
- Place water near the site to wash up spills that come in contact with skin and clothing. Spray (shower) and eye wash station (eye wash station) is also very good prepared there.
- Prepare baking soda or agricultural lime to neutralize the acid spilled soil.
Step 4. Put the acid in a plastic bucket or jar filled with water
Unlike metals, plastics tend to be acid-resistant at these concentrations. To avoid a strong acid reaction, first pour the water into the bucket, then slowly add the acid. Always follow the manufacturer's instructions listed on the product packaging before you use the guidelines below. The ratio of ingredients below is usually suitable for some types of products, but not all:
- Sulfamic acid: 450 grams of powder or acid crystals for every 4 liters of hot water (120 grams for 1 liter of water).
- Phosphoric acid: dilute the acid to 20-40%.
- Muriatic acid: mix 3-4 parts water with 1 part acid, or follow the directions on the package to get a concentration of 10% (15% if the concrete is smooth and hard).
- This solution is used to etch concrete. If you only want to get rid of mineral deposits (swelling), use a much thinner ratio (10:1 or 16:1 for muriatic acid).
Part 2 of 3: Applying Acid
Step 1. Spray the entire area with water through the hose
Spray water on the concrete until it is damp, but not soggy. Wet also objects that are around it, such as trees, doors, walls, door frames, and carpets. Get rid of furniture that is near the area.
- Concrete should be wet at all times. To treat large areas, separate the area into sections, or periodically spray the hose with water to keep it from drying out.
- Protect the asphalt, gypsum (drywall), and tarmac (a mixture of gravel and asphalt) with a plastic sheet or other protective material.
Step 2. Sprinkle the acid
Use a plastic sprinkler to spread the acid on the concrete (within close proximity to the floor when doing so). Do it in small sections, starting in a slightly hidden area. Plastic shells can corrode with acid (sometimes within 1 hour) so you'll need to have some spare pads ready. Read the directions on the product packaging for how much acid to add, or use these guidelines:
- Sulfamic acid: 4 liters to handle 90 m2 concrete (1 liter for 28 m2).
- Phosphoric acid: 4 liters to handle 150–760 m2 (1 liter for 45–250 m2) if used to remove mineral deposits.
- Muriatic acid: 4 liters to handle 14 m2 (1 liter for 5 m2).
Step 3. Rub acid on the concrete
As soon as the acid is removed, scrub with a long-handled brush to evenly distribute the acid. To deal with large areas, you may need a friend. One person runs the floor machine, while the other rubs acid into corners and walls.
Keep the floor and everything around it dry when you apply the acid. Maybe you should spray it often
Step 4. Let the acid sit there for a few minutes
Wait about 5 to 10 minutes for the acid to etch the concrete. If you just want to get rid of the white mineral deposits, wait for the deposits to lift off the concrete (it usually only takes a few minutes).
Part 3 of 3: Doing Cleaning
Step 1. Rinse everything thoroughly
Before the acid dries, scrub away any remaining residue using a long-handled brush while flushing it with plenty of water. Concrete can damage if you let the acid sit there for too long.
Step 2. Neutralize the acid
Mix 1 cup of baking soda, agricultural lime, or household ammonia into 4 liters of water (about 250 ml in 4 liters of water), or follow the directions on the product package to neutralize the acid. Rub this mixture into the concrete and let it sit for at least 10 minutes to neutralize all the acid. Pay special attention to edges and low sections of concrete.
At this point, the etched concrete will have a uniform texture like sandpaper with medium roughness. If the condition of the concrete is smoother than this, or there are still white mineral deposits, apply the acid again
Step 3. Rinse the concrete several times
Even after the acid has been neutralized, the liquid remaining on the concrete surface can form a white, powdery residue after drying. Spray the concrete with water, then scrub it, and repeat this process a few times to prevent this from happening. Suck up the last rinse water with a wet vacuum cleaner (shop vacuum), or sweep the water down the drain.
- Use a water hose to rinse off the acid, not a pressure washer. This tool actually makes the acid seep deep into the concrete.
- To be on the safe side, test the last rinse water with a pH meter. If the pH is below 6.0, it means that the concrete still contains a lot of acid and needs to be rinsed again. (Although it's rare, if the pH is over 9.0, you've used too much acid neutralizing agent.)
Step 4. Wipe off any remaining acid
If there is any remaining acid solution, slowly pour the acid into a large bucket that has been given the neutralizing solution you used earlier. Gently stir the acid and neutralizer mixture until it no longer sizzles. Once neutralized, pour the acid into a sink or drain. Spray hose water on all equipment or clothing that has come into contact with the acid.
If the remaining pure acid is no longer used, you can dispose of it the same way. Acid placed in storage can pose a serious hazard due to the risk of spilling or producing corrosive fumes
Tips
- Ask someone for help if possible. This process can be done more quickly if done by 2 people. You brush the floor, and your friend keeps spraying hose water.
- Allow the concrete to dry for at least 2 days before you apply anything (the wait time may be longer if conditions are damp, cold, or poorly ventilated). Even if the surface appears dry, the moisture beneath the surface can damage the coating applied to the concrete.
Warning
- Never pour water on acid. Always add acid to the water to prevent harmful acid splashes. After that, stir the mixture slowly.
- Keep the entire area wet as you go through this process. This is useful so that the acid doesn't damage things. Muriatic acid not only eats away at concrete, it can damage wood, metal, and various synthetic materials such as carpet.
- Keep children and pets away from the area.
