What Is Pool Plaster Mottling? And How To Fix It

Written by Michael Dean
September 21, 2023

pool plaster mottling

While looking after and maintaining the pool, a common issue pool owners encounter is discoloration of the pool plaster, also called “mottling.” The discoloration caused by mottling is unsightly, so you may wonder if there is a way to fix this issue. In this article, I will provide some fixes for these unflattering blotches on your pool plaster and go over what causes it in the first place.

Main Takeaways

  • Mottling is a patterned discoloration of your pool’s walls.
  • It can occur due to incorrect trowelling, uneven application, or improper water balancing.
  • Acid washing and blow torching can be used to fix mottling.
  • Replastering is a last resort when your pool displays signs of wear and tear that are more expensive to repair than replace.

What Is Pool Plaster Mottling?

Pool plaster mottling generally looks like speckled discoloration on your pool’s plaster. It is a common issue for pool owners, and you’ll want to learn how to fix and avoid it to keep your pool looking pristine.

pool plaster mottling

Mottling can range from mild to severe and can affect all types of pool plaster finishes, making it an unpredictable issue that can affect any pool at any time. It is worth noting that mottling is primarily an aesthetic drawback that does not impact your pool’s lifespan; therefore, there is no cause for alarm. That said, it is still worth understanding why mottling occurs and what can be done to prevent or remove it.

What Causes Mottling?

Here are some common causes of pool plaster mottling, some of which can be caused by a bad pool plaster job.

Grey Mottling

Mottling can occur when a high amount of calcium chloride is added to a plaster mix. This type of mottling is called “grey mottling.” This type of mottling has a slightly different “grey” appearance than other types of mottled plaster.

Calcium Chloride In Plaster Mix Is an Issue

In my opinion, the plasterers need to produce a consistent finish and stop contaminating the mixes. By adding calcium chloride to the mix design, when in fact it is not required, they are intentionally contaminating the cement with salts that artificially and prematurely harden the cement causing a surface glaze, trowel burn, and moisture entrapment, cumulatively called mottling.

Yet almost every plaster truck in the country has a bag of calcium chloride by the mixer. They call it making the mix hot, so it will finish quicker. In California, the term for the mixer man is the “Hotty.” It’s always upset me as a materials scientist to have your products intentionally compromised.

The problem is largely self-induced and we need to hold the plaster company responsible for its actions. Stop using calcium chloride in pool plaster!

It also damages the color. Read any color manufacturers label and they all say the same thing, “Do not use with calcium chloride salts.” For me personally, plasterers consistently compromised my Silicone Shield plaster product 20 years ago with their calcium chloride additions and I never forgave them.

Anyways, let’s get back to some other causes of mottling.

Incorrect Construction

Mottling is generally an indication of trapped moisture at the time of construction, appearing anywhere between the first day that the pool is filled or during the first 4 to 5 months of use. High pH or alkalinity during the initial hydration period can lead to calcium deposits over the finish, sealing in excess moisture and causing the plaster to become discolored.

Uneven Application of Plaster

Plaster is hand-troweled, so there is always room for error during the plaster application process, especially if you do not hire a professional to take care of this part of the process for you. If the plaster is applied at a thickness that is either over or under recommended specifications, you risk mottling forming at later stages of the curing process.

Unbalanced Chemistry

There is not a lot of research to support this claim, but there are some experts who state that unbalanced water during the start-up process can cause mottling.

How To Fix Mottling

There are a few different ways you can try to fix pool plaster mottling, depending on the problem’s severity and the type of pool.


Blow-torching mottled areas is a common method used to expel trapped moisture. This high-heat method is generally employed for areas with an extremely dark color. However, while this method may lighten the area, it will not remove the discoloration altogether. This method also weakens the plaster and may do more harm than good, costing you much more in the long run.

Acid Wash

Acid washing the surface of your pool is another common method to deal with pool plaster mottling. To do this, dilute muriatic acid in water and spray the solution onto the surface of the drained pool. Then, you should scrub the affected area to remove the surface stains. Repeat this process until you achieve the desired results.

In quartz pool finishes, acid washes can help reduce further instances of mottling. However, for other pool finishes, acid washing can cause the plaster to become “etched,” which will only make the mottling more pronounced. Acid washing can eat away at your plaster finishes and may even lead you to have to resurface your pool in extreme cases.

Replaster The Pool

While the methods suggested above seem like great fixes, they are temporary and may do more harm than good.

In reality, the ONLY way to truly fix mottling is to resurface the pool. Replastering a pool can be costly and time-consuming, so only do this as a last resort. Mottling may not look the best, but it doesn’t affect the integrity of the pool, so ideally, you should leave it alone until it comes time to resurface the pool due to excessive staining or other hard-to-fix issues.

How To Avoid Mottling

It is difficult to avoid mottling altogether. However, here are some of my top tips to prevent mottling as best as you can:

  • After plastering, ensure the water is balanced correctly when the pool is filled for the first time.
  • Ensure a smooth plaster coat. The chances of mottling will be greatly reduced with better and finer craftsmanship during the plastering process.
  • Ask your pool builder how much calcium chloride is added to their plaster mix. Ideally, it’s as low as possible.

When To Replaster The Pool

Unfortunately, there will be a time when you will need to replaster the entire pool to fix mottling or other issues. But how do you know when is the right time to replaster the pool?

As a general rule of thumb, pool owners reapply a fresh coat of plaster every ten years. So, if it’s been around ten years since you last plastered your pool, the time may be ripe to do so.

You should also consider replacing your plaster earlier if you notice that acid washing or blow-torching is no longer reducing the plaster’s stains. Or if replastering is leading darker areas to appear, leading to an uneven surface and feel.

Rough plaster is another reason to replaster your pool. Rough plaster not only feels terrible on your skin and can cause snags on swimsuits, but it is also a huge breeding ground for algae. However, If only a small area of plaster is rough, you may be able to get away with chipping the section away and replastering the small section with epoxy or underwater putty.

Another reason you should consider replastering is when the pool’s walls are thinning or chipping. When dealing with such concerns, to be on the safe side, consult a pool professional on whether replacement is the right step to take before going ahead.

Need Some Maintenance Help?

Send me a message! I can answer any of your pool maintenance, equipment, or other questions.

Bottom Line

I hope this clears up any worries regarding the occurrence of mottling in your pool. Fortunately, in reality, mottling isn’t as bad as it looks! While there are some temporary fixes to deal with discoloration of this sort, the only proper fix is to replaster the whole pool.

Shoot me a question if you have any further questions about pool plaster discoloration. I’ll be happy to help out! To learn more about other common plaster issues, check out my article on plaster delamination.

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