How to Acid Wash a Swimming Pool

by Pool Builders on 07-13-2013 in Articles

Swimming pools are continually attacked by algae. Algae enter your swimming pool from the wind, rain, and even contaminated swim suits. Typically, the normal recommended amounts of chlorine or bromine are able to kill algae when it makes contact with the water. However, if the pool has been stagnant for a long period of time, or if the winterizing process was not performed properly the algae may take over. If this happens, the swimming pool will likely turn to a dark black or green color and you won't be able to even see the bottom. The pool will likely require an acid wash to restore it to its former beauty and cleanliness. As a general rule of thumb if you can see the bottom of a swimming pool, you can usually bring it back with chemicals, vacuuming, and filtering. If the floor is not visible, the cost of chemicals and electricity for filtering will far exceed the cost of an acid wash. Filtering and chemicals will also take a very long time to kill and remove the algae, thus making an acid wash much more cost and time effective. If algae seems to be able to grow overnight even with proper chemical maintenance, changing the water and acid washing the plaster surface of your swimming pool will give you an algae free summer.

An acid wash will strip a tiny layer of plaster off of the swimming pool, thereby exposing the newer, cleaner plaster underneath. It shouldn't be an annual custom since it doing so will strip all of the plaster away and create a need to completely re-plaster the swimming pool. Most plaster coats of white coat or marcite are more than ½ an inch thick, so the occasional acid wash isn't harmful to the swimming pool. Acid washes can also be used to bring out a brighter and cleaner finish for your swimming pool, and not necessarily due to an algae crisis. A swimming pool always comes out brighter and cleaner from an acid wash.

The acid used for acid washing is Muriatic acid. Muriatic acid is more commonly known as Hydrochloric acid with the chemical symbol HCl. It is found in highly concentrated forms at the average swimming pool stores, and the dangers cannot be emphasized enough. Acid is a dangerous substance. Anyone who acid washes their pool must take proper precautions to assure safe handling of the acid and to minimize exposure. Wear proper protective clothing, with an air purifying respirator designed for acid fumes. Wear safety goggles not safety glasses, rubber gloves, and boots. Pool company personnel are specially trained in acid application and acid safety.

The first thing to do for an acid wash is to drain the pool completely. As the pool drains wash and scrub it down as necessary to remove all algae and leaves. When the pool is empty and dry, it is time to begin the acid wash. Be sure to don all of your protective clothing and equipment.

To obtain the proper acid mixture of 50/50, add one gallon of acid to one gallon of water in a large flower watering can (Always add acid to water, never the other way around as you will risk an explosion). Using a running garden hose without a nozzle, wet down the walls of your pool. Keep the hose running continuously while cleaning the pool. Pour the acid down the swimming pool sides, from top to bottom, one ten foot section at a time (start at the deep end, working your way to the shallow end). Do not allow the acid to sit on the plaster for longer than thirty seconds as this will cause damage. Use an acid brush to scrub the surfaces of the pool and move the acid around. After scrubbing, rinse thoroughly and quickly. Take care to remove all of the acid from the pool walls. Any acid left behind will continue to etch the plaster. If the 50/50 acid mixture is unable to remove all of the stains, you can increase the concentration, or the length of time the acid remains on the walls, and/or scrub harder.

After the acid wash, all of the acid will have collected on the bottom of the swimming pool. It will look like a foamy acid puddle and will need to be neutralized before being pumped out. In order to neutralize the acid use 2 pounds of soda ash for every gallon of acid that has been used. Soda ash can be found at pool supply stores. Spread the soda ash over the puddle while stirring with a pool brush on a pole. Once the acid is neutralized, you may use swimming pool testing strips to measure the pH and check if the acid is neutral. Do not use a chemical testing kit as the acid may damage the plastic testing container. Neutral is on the pH scale of 7. Anything below 7 is still acidic. Once the acid is neutral, a submersible pump may be used to pump the neutralized acid out of the pool. Be careful where the neutralized acid is pumped to as it may destroy plants and kill fish, frogs, or any other living things. Once the acid is pumped out, rinse the bottom of the pool again.

Don't rush the acid job. Do the job slowly and safely. Acid fumes can be very strong and very dangerous. Be sure your respirator can block all acid fumes. Make sure the safety goggles fully protect your eyes. Be sure to wear old pants, a long sleeve shirt, rubber gloves and rubber boots. Spray yourself off before exiting the pool. Always have a second person nearby when acid washing the pool to help with any emergencies including calling 911 should the need arise. If acid falls into the mouth, eye, ear, or on exposed skin, rinse with the hose for 15 minutes.

If the swimming pool is lined with vinyl, do not use acid. Instead use detergents and elbow grease to remove the algae from the walls. Try not drain the liner pool completely as liner is difficult to reset into place. There is also the small possibility of the pool walls collapsing.

It is strongly recommended that you hire a professional to perform the acid wash for you, rather than do it yourself. After a thorough acid wash, the swimming pool will look like brand new. With proper chemical maintenance from this point forward, the pool should not have any further significant algae problems.

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