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Also called a drain & clean, an acid wash becomes necessary if the pool has turned into the "black lagoon". This may occur if the winterizing process is not done properly, or if the pool has been stagnant for a period of time so that algae has taken over, and is also filled with leaves and debris.
A general rule of thumb for determining the need for an acid wash is - if you can see the bottom of the shallow end floor, then you can [probably] bring it back with chemicals, cleaning and [effective] filtering. If the floor is not visible, the cost of the chemicals and labor may exceed the acid wash charge, and take much longer. Also, extensive algae blooms or neglect will stain plastered pools, making an acid wash desirable.
An acid wash is the purposeful stripping of a thin layer of plaster, exposing fresh plaster beneath. Therefore, it is not advised to make it an annual custom, which will accelerate the need for re-plastering. Most plaster coats (sometimes called whitecoat or marcite) are in excess of 1/2", so a careful acid wash every 5 years won’t hurt.
You may also decide on an acid wash not because of swamp conditions, but just to bring out a brighter, whiter finish. Mineral stains and/or deposits, chlorine stains, even dirt stains... an acid wash is always a dramatic aesthetic improvement to a plastered pool or fountain.
If your pool has had years of algae blooms, and if your pool seems to grow algae overnight or just bloom very easily, and you’ve tried everything else - changing the water and acid washing the surfaces can give you an algae-free summer. Changing the filter media at the same time is also recommended.
Muriatic Acid is a dangerous substance. Alternatives such as Acid Magic exist that have low fumes and are less hazardous to work with. All personnel should be trained in safety around a drained pool, and those doing the acid wash should wear protective clothing and breathing mask rated for acids. To protect our environment, the acid/water waste should be neutralized with pH increaser prior to being pumped to a safe location.
If you decide to drain and clean your own pool, make sure to remove the hydrostatic relief plugs as soon as possible, and that the water is pumped to a distant location, or into a storm drain, so that it won’t return beneath the pool. If your pool sits in a topographically depressed area, or if you have a high water table, or heavy recent rainfall, be extra careful with managing the hydrostatic pressure under the pool. You may also need to check with local water authorities for waste water discharge regulations in your area. If pumping across soil, move the discharge hose regularly as needed, to avoid oversaturation and erosion.
Acid Washing Supplies Needed:
STEP 1: DRAIN & CLEAN With a main drain in the pool and a multiport valve on the filter, you can drain most of the pool water with your filter pump, but it will lose prime with 1-3 feet of water still in the deep end. You can use a submersible pump to finish the job, like the Water Wizard, which fits in a main drain pot. For bad pool clean-ups however, go to your local rental shop at 7 am, and rent a 2” or 3” Trash Pump, with a 20’ long suction hose, to reach the main drain, and as much discharge hose as you need, 50’ or 100’ usually. Stop at the gas station and fill up a gas can for refueling.
As you drain the pool, wash it down (scrub or use a push broom) to remove all algae and leaves. If you let the algae (and other gunk) dry on the surface, it becomes twice as difficult to remove. Spray the surface with a water nozzle (walls, then floor) as the water level drops, before it dries into a baked-on finish. When all of the water is removed, bag up all leaves and debris in the pool's bottom. Rinse again, remove the main drain cover so that you can pump out all of the water, to begin acid washing the bowl, or the area around the main drain.
When the pool is clean and completely empty, you can begin to acid wash the plaster, in the deep end. Put on protective clothing and rubber boots, goggles and wear a breathing mask designed for acid fumes.
STEP 2: POUR THE ACID Add equal parts acid or Acid Magic to water in a flower watering can with a long pouring spout. (Always add acid to water, never the other way around). A weaker solution than 1:1 can be used, try to use as little acid as possible, to avoid damage to the plaster. If you pressure wash the pool first, you will reduce the amount of acid needed.
Moisten the wall with a hose before you pour acid; avoid pouring the acid mixture on dry surfaces. Keep the hose running at all times, without a nozzle, although you can use a second hose with a nozzle to make spraying easier. Do not use a pressure washer to rinse the acid however, just a regular garden hose with a nozzle.
Start the acid wash in the deep end area around the drain, because it will quickly fill up with wastewater. Pour in a circular pattern, and have a helper follow behind with a stiff push broom. Then rinse thoroughly and add a few lbs of pH increaser to the wastewater in the bowl to neutralize the acid.
Starting in the shallow end steps, pour the acid/water mixture down the wall, from top to bottom, one section at a time, until finishing with the deep end walls. Do not allow the acid to sit on the plaster for very long. 30-60 seconds for horizontal surfaces, and 60-90 seconds for walls. Use an acid brush to scrub stained surfaces and to move the acid around. Rinse quickly and thoroughly.
Make sure acid is rinsed completely, as it will continue to etch the plaster. Also, work to prevent the acid from wearing a channel path on the floor from shallow end to deep end. Keep pushing the acid around, spreading it with water, the hose and/or a large push broom or acid brush, to keep it moving while diluting the mixture. If you let it sit too long on the steps, or allow the acid/rinse water to run in one path only, it can wear these areas thin and rough.
If the 50/50 mixture isn't strong enough, you can increase the acid strength or the hang time (before rinsing), or you can scrub harder. Pools can also be acid washed a second time with a weaker mixture, or ‘spot wash’ areas that are still stained.
Remember that you don't want to damage or "burn" the plaster, by using too strong of a mix, or letting it sit too long. Horizontal surfaces like steps or swim outs need a triple-rinse to be sure all of the acid is washed away. Acid does not deplete itself; it continues to corrode, until completely diluted and washed off.
STEP 3: NEUTRALIZE After the acid wash, the bottom of the pool will be filled with a foamy, acid puddle. This needs to be neutralized before pumping out. It is best to neutralize as you go, using about 2 lbs of soda ash per 1 gallon of acid used, or do a base demand test. Broadcast the soda ash (pH increaser) over the wastewater puddle while stirring with a pool brush and your pool pole. Use your pH test kit to determine when you have returned the water to as close to neutral (7.0) as possible.
STEP 4: PUMP OUT WASTE WATER Use a small submersible pump with a hose to pump out the remaining acid water. Run the discharge hose up the shallow end floor if your pump is having trouble lifting water out of the deep end. Be careful where you pump the acid wash waste water. Even if properly neutralized, it can destroy plants or harm fish and animals.
Rinse the deep end bowl with water again, and if needed, re-pour the bowl to clean up well around the drain, being careful with the strength, and diluting with lots of rinse water within 60 seconds. After the bowl is cleaned, begin to fill the pool, remembering to resecure the main drain cover(s) tightly and close any hydrostatic relief valves or plugs that were opened.
ACID WASH SAFETY: Don't rush the job and be safe. The fumes can be very strong, and very dangerous. Be sure to wear a respirator that will block muriatic acid fumes, goggles or safety glasses and protective clothing. Wear old shoes, or rubber boots. Spray off before exiting the pool.
Transporting muriatic acid from the hardware store to the house can be hazardous also. Secure the load in the vehicle. Always have a second person nearby when acid washing the pool. If acid drops enter the mouth or eye, rinse with the hose for 15 mins, without a nozzle on it. Acid on the skin won't usually burn too much, just rinse quickly, for 30 seconds. Also be safe to not allow any other chemicals to come into contact with the acid, or a dangerous reaction could result.
Falling into empty pools is another hazard of acid washing. Take care that the deck area around the pool is clear and free of trip hazards, and that everyone is aware of the danger of falling into an empty pool.
VINYL LINERS: If your pool is vinyl lined, acid is NOT used to clean vinyl. Small amounts of detergents and good 'ol elbow grease will remove the "slime". If drained completely, inground pool liners must then be reset with a vacuum to ensure proper fit during filling. For this reason, liner pools are normally not drained completely if it can be helped, as there can be problems getting the liner to reset properly. Half-draining (to within 6” above shallow end floor) and refilling, while removing leaves and debris can often make a large difference in the speed and success of turning around a vinyl lined ‘black lagoon’ pool. Repeat if necessary.
ACID WASH PRICES: Complete drain & clean charges average $750 for a residential pool. Higher costs may be seen for large amounts of debris in the pool, excessive neglect, or larger sized pools. Lower costs will be realized for clean or empty pools, or localized acid washing.
COST OF WATER: If your water is from a well, you may elect to refill the pool with trucked-in water. Expect to pay about $200 per 5,000 gallons for this luxury. Most wells can fill a pool without a problem or worry about 'burning out the pumps' or 'running the well dry', but if concerned you can fill partially or fully from trucked-in water. If you refill from the hose, water costs are a usually $3-$5 per thousand gallons. Contact your Water Authority to let them know you are filling the pool and they may not charge you for the sewer, only the water portion of the bill.