Acid Washing Pool Plaster: Critical Part of the Installation Process

Written by Michael Dean
June 28, 2023

acid washing pool plaster

If you’ve been reading up on the pool plastering process, you’ve probably encountered the term “acid wash” and wondered what it is and why it is important. Acid washing sounds scary, but it is a critical part of the pool plaster installation process.

In this article, I will explain why acid washing the pool before plastering is needed, what happens if you don’t, and how to safely acid wash pool plaster.

Main Takeaways

  • Acid washing your pool is part of the pool plaster prep process. It is also part of the initial pool construction process for new swimming pools.
  • Acid washing is important because it removes stubborn stains and harmful bacteria.
  • Not acid washing before applying new plaster can result in poor plaster curing and a shortened lifespan.
  • Always wear protective gear when working with acid, especially thick acid-resistant gloves.

Where Acid Washing Fits Into the Pool Plaster Process

Plastering a pool is a significant and, at times, messy job. There are a lot of steps that make up the pool plaster process (you can read my What Is Pool Plaster? guide if you’re unfamiliar with the material). But before you can begin applying the bond coat and start plastering, it is essential that you properly prep the pool.

Acid washing is an essential step in the prep process for plastering. After draining the pool and relieving the hydrostatic pressure, you must sand and chip away any blemishes. When the pool’s surfaces have been adequately inspected and fixed, use muriatic acid to acid wash the pool.

After a couple of minutes, wash off the acid using a hosepipe and leave it to dry before you apply the bond coat and start plastering.

Why is Acid Washing Important?

Washing your pool with acid before replastering a pool is vital so that you’re not starting over a problematic surface. If there are any issues underneath, once you’ve added the new layer of plaster, the only option is to remove it and start the entire costly process again.

Acid washing is used to treat different stains and bacteria before the new layer of plaster can be applied. It is also used in the initial pool construction process to remove silt and grime to reveal a clean pool plaster finish.

Prevent Magnesium Stains

High magnesium levels can affect the pool from rusty taps when filling up the pool water. Pool water with high magnesium levels can cause problems like plaster discoloration, typically pink, black, and red. Discoloration can’t be removed with regular pool cleaners. 

If you don’t wash these stains before applying the new layer of pool plaster, the old stains will start to discolor the new layer of plaster.

Remove Bacteria

Harmful bacteria can grow in between the cracks and holes of the old pool plaster. If left to grow under the new plaster, you could end up with rotting plaster, bubbling plaster, or stained plaster. Acid washing the plaster before laying down a new layer will remove the bacteria that causes these issues.

Prevent Algae Growth

Lack of proper cleaning, poor pH balance, and poor pool circulation can all lead to algae growth. The longer algae is left untreated, the more difficult it is to clean.

Black algae, in particular, is very dangerous if left to grow under the surface of the new pool plaster. It develops a protective layer that is difficult to clean. With its new roots that can sprout into the plaster, it is absolutely crucial that you acid wash your pool before new plaster is applied. 

Hard Water

Hard water can lead to calcium scales along the pool water line. You’ll know if you have a hard water problem if your pool water is cloudy even after you have thoroughly cleaned it. A calcium build-up can create holes in pool plaster, so an acid wash is essential in removing this scum before new pool plaster is applied.

What Happens If You Don’t Acid Wash When Applying Pool Plaster?

Acid washing before applying new plaster is crucial to the longevity of the new plaster. An acid wash involves applying a layer of muriatic acid on the pool surfaces. The acid helps remove any bacteria that may be trapped in the cracks or holes in the old plaster. If you don’t acid wash the old plaster, you risk bacteria and mold growing underneath your new layer of plaster.

When bacteria, algae, and mold grow underneath the new player of plaster, it damages the entire pool structure and can cause the plaster to bubble and rot from the inside out. The life of your pool will be drastically shortened if this happens – and replacing your entire swimming pool will be much more expensive than the extra time it takes to acid wash your pool.

Other problems that may occur by not acid washing your old plaster include dirty water, clogged filters, and pool fittings. Washing your pool with acid helps clean away all the loose plaster dust and debris left in the pool. If you do not acid wash the pool, this dust and debris can cause damage to the pool by clogging the fittings and pool filter.

Not doing an acid wash can lower the life expectancy of your pool plaster and leave you sitting with expensive damages that need to be repaired later on.

How to Safely Acid Wash Your Pool For Pool Plastering

Acid washing can be a dangerous process if done incorrectly. That is why you must follow all instructions and handle the acid with care. Here are some of my top tips to safely acid wash your pool for pool plastering.

Safety First

Safety is the main thing to remember when acid washing your pool. The acid mixture is very strong and can cause harm if it makes contact with any part of your skin. When acid washing, wear protective clothing from head to toe. Wear long clothing that covers your limbs, close-toed shoes, a pair of thick gloves, and protective glasses.

Mix According To Instructions

When mixing the acid wash mixture, you will use a ratio of 1 gallon of water and 1 gallon of muriatic acid. You must pour the acid into the water, not the water into the acid. If you don’t do it in this order, you may be at risk for chemical burns as the mixture reacts by boiling.

Work In Small Sections

Work on small, manageable segments at a time. Apply the acid solution and let it sit on the plaster surface for about a minute before you start scrubbing. When you have finished scrubbing the entire pool, rinse the acid towards the drain.


Pouring your acid solution straight into the sewer spells disaster. You must neutralize the acid before you dump it and ruin your drainage hose or pump, not to mention the harm it poses to the environment. So for every gallon of acid, put in two pounds of powder soda ash. This will make it safe to dispose of the acid solution.

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Bottom Line

Acid washing your pool before installing new plaster is essential so that you avoid bacteria growing under your newly installed pool plaster. It may seem daunting, but with the proper safety precautions and knowledge, you will safely and effectively clean your pool’s surfaces, prepping them for the plaster installation.

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