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What is fiberglass "chalking", and what causes it?

Fiberglass pools and spas often turn white, with a hazy film that looks like chalk or calcium scale. But "chalking" is actually the deterioration and oxidation of the fiberglass gel coat.



If you have a white haze in a fiberglass pool or spa, there's a good chance it is not calcium scale. The more common condition is what we call "chalking", which is caused by aggressive water degrading the gel coat enough so that the polymers within it can be oxidized by chlorine (or another oxidizer). This oxidation turns the gel coat white.

Aggressive water degrades fiberglass gel coats

When the water is aggressive, (LSI below -0.30) it looks for calcium to dissolve. In a cementitious pool, calcium is everywhere because it's part of the cement surface. But in a fiberglass pool, where is water able to find calcium? 

If calcium cannot be found, materials are attacked by the low-LSI water. Eventually, the water is capable of degrading the surface of the fiberglass enough that the gel coat becomes permeable, and chlorinated water can interact with the polymers within. We don't know the exact chemistry of these polymers, but we have abundant field evidence that when they are exposed to chlorinated water for long enough, they turn white. This is a byproduct of oxidation.

In other words, chalking is not a form of calcium scale. It's evidence of aggressive water permanently damaging the fiberglass surface. Once chalking begins, there is no chemical remedy we know of. The pool would need to be refurbished by a fiberglass company that can sand off the damage and apply a new gel coat.  This is expensive, but it can be done.

How to tell if the white haze is chalking or calcium

fiberglass pool chalking-1

An easy way to know if what you're looking at is scale or chalking is to dilute some acid on a rag and wipe the affected area directly. Always wear protective gear (at minimum, this means gloves and glasses) when handling acid. If the white cleans off and is easily removed, it's most likely calcium carbonate, and that's great news! You can just follow our procedure for softening and removing scale with SC-1000.

But if it's chalking, the damage is done. We're sorry to hear it. Unfortunately, this is a VERY common problem and is the result of unbalanced water.

The importance of LSI balance in all pools, but especially fiberglass pools

Water cares about equilibrium, and that equilibrium is measured using the Langelier Saturation Index (LSI). If your water is aggressive (red LSI on the Orenda Calculator™), damage to the pool surface and its equipment will occur. How fast this occurs depends on too many variables to name, but rest assured, if the water is aggressive long enough, damage occurs.

We find it odd that chemistry recommendations for fiberglass pools limit calcium hardness to 200 ppm or less. We have seen documentation instructing pool owners to not exceed 150 ppm of calcium hardness!  We wholeheartedly disagree with this advice, because we understand the LSI and its importance.  It is very difficult (if not mathematically impossible) to maintain LSI balance with such low calcium levels, especially in colder water temperatures

Use the Orenda Calculator™ for yourself and see what we mean.  Plug in 150 calcium and try to balance the LSI. Most pools would need well over 100 ppm total alkalinity to offset such low calcium hardness...but that, in turn, raises the pH ceiling to a point where scale is virtually inevitable. It doesn't make sense from a water balance perspective.


Chalking is not calcium buildup or scale. It's permanent damage to the fiberglass material due to aggressive water weakening the gel coat, which then gets oxidized by chlorinated water and becomes white. The way to avoid this is to maintain LSI balance year-round, which is our first pillar of proactive pool care.