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

Fiberglass pools and spas can turn white with a hazy film that looks like chalk or calcium scale. Why?

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 currently being studied in a lab and by some forward-thinking fiberglass pool manufacturers. We have been in direct contact with them and are grateful for them leading the charge in figuring out the chalking problem.

This article is being revised from our previous version,  where we published our theory that low-LSI (aggressive) water degrades fiberglass gel coats and chlorine oxidizes it, turning it white.  But as preliminary lab results have shown, the white material that we call 'chalking' seems to be a form of calcium oxide (CaO) that is chemically bonding to the gel coat itself. And not just on the gel coat, but within it. 

This is surprising, to say the least, but when the molecular structure of a gel coat was explained to us, it makes more sense.

Gel coat molecules and crosslinking polymerization

Fair warning, this explanation is simplified, not overly specific. The chemistry of gel coats is more complicated than this article intends. 

First, watch this video explaining gel coats from River Pools:

A releasing agent is applied to the mold prior to spraying the gel coat on. When the gel coat is sprayed, it cannot bond and crosslink with the releasing agent. So it bonds with the gel coat molecules adjacent and behind it. While this makes the first layer strong, it plays an important role in what we are seeing in the lab results testing chalking fiberglass samples.

The more crosslinking, the less flexible and harder the material. This also makes the gel coat more brittle. Hardness is tested using a Barcol Hardness tester; a device that drives a pin into a material and measures how hard that material is. While this is news to us, fiberglass companies have known this for a long time.

So in summary, the first layer of gelcoat applied to the mold is the strongest, because it can only crosslink with the adjacent molecules and those behind it. This makes these bonds more dense.  And speaking of dense, you might notice on fiberglass pools that have chalking issues that the edges of steps and benches may not be chalking as much as the flatter surfaces around them, or the inside radiuses between the steps (see the photo below):

Notice the top edge of the steps (outside radius) are darker than the rest of the areas, and the base of each step (inside radius) is lighter than the rest of the areas. Our best theory so far is that these color variances have to do with the density of these crosslinked polymers.

While the top of the step is an outside radius in the pool, you need to remember that this pool was shot onto a mold, which is the inverse of this pool. So that top edge of the step is actually an inside radius in the manufacturing process, which means a denser bonding in those areas. The opposite is true for the base of each step. These areas are the least dense areas of the gel coat, which we theorize is why there is more chalking occurring there.

UPDATE (April 2025): Calcium oxide is bonding with the polymers.  But why?

While we are not doing these lab results ourselves, a manufacturer we are working closely with has invested enormous amounts of time and resources to solving this problem, and we are grateful to them!  They've asked to remain anonymous for now until all the results are done and this problem is solved.

The lab results have shown that inside these gel coats (yes, inside them), calcium oxide is being found. Calcium oxide (CaO) is not the same as calcium carbonate (CaCO₃). We honestly don't know why this is happening (yet). It appears that this calcium oxide, in certain chemistry conditions, is able to pass through the surface layer of polymerized and crosslinked gelcoat, and bond to something deeper.  We don't know what, and we don't know why.  But the labs have clearly shown that calcium oxide is in there.

This helps explain why muriatic acid does not clean up chalking. We can distinguish this from carbonate scale, because acid would dissolve scale quickly and remove it. And yet acid does not seem to affect this.  Perhaps the acid is unable to reach it inside the gel coat? Or CaO reacts less with acid than CaCO₃?  We don't yet know.

What we know so far

The tests are comparing calcium hardness levels at various pH levels, and various chlorine levels. The results seem to be all over the place, but there are a few common denominators that we will discuss in a moment.  Primarily, chalking is more likely when pH and chlorine levels are higher than normal, and calcium is present that is unchelated (or sequestered). 

As of now, it appears there might be at least two ways to manage water that prevents chalking. 

The first common denominator is in all scenarios of the lab tests, SC-1000 proved to be very effective at preventing chalking.

Chelating calcium with SC-1000 prevents it from bonding to other things, so this makes sense. Apparently the calcium level could be as high as 1000 ppm if SC-1000 was used, and no chalking occurred in the tests.  Without SC-1000 (or comparable sequestering agent), chalking occurred in most samples that exceeded 120 ppm calcium hardness.

We do not yet know what role pH plays in this, nor do we know the role chlorine plays in it. Time will tell as more tests are conducted and the results come back.  As of right now, there are two recommendations we will share with you.

1. The manufacturer(s) recognize that calcium oxide (CaO) is what is causing this issue to turn white and appear like chalk. So they recommend keeping the pH from going too high (but an exact number isn't known yet), and keeping calcium hardness low (around 120 ppm or less). They also recommend using SC-1000 or a non-phosphate sequestering agent to keep calcium in solution, especially if you have calcium levels that exceed 120 ppm.
   1. In our ongoing discussions with the fiberglass manufacturer doing these tests, it is clear that these recommendations are well intentioned.
   2. Our concern is these numbers force a low LSI violation, and furthermore, the pH will naturally rise. And while it's true that fiberglass pools contain no calcium for water to steal, AND that fiberglass is incredibly resilient against aggressive water, we do not yet know the long-term consequences of these numbers. Our assumption is aggressive water will remain aggressive, and could impact pool equipment and other factors. But as of now, it doesn't seem to attack the fiberglass itself...so that's good news, and it pleasantly surprised us.
   3. We do like the recommendation of chelating calcium with SC-1000 (but of course, we're biased there).
2. Our recommendations, based on the latest information, is to chelate the calcium using SC-1000, contain pH and keep the pH ceiling below 8.10 (meaning 8.09 or less), and having enough calcium to maintain LSI balance at all times.
   1. We have many fiberglass pool owners that have contacted us and confirmed this strategy has worked well and no chalking has occurred for them.  Some of these customers had severe chalking, had their pools refurbished, then implemented our protocol, and so far, no chalking has returned in their pools. 

How to tell if the white haze is chalking or calcium

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 may require refurbishing the pool.

Conclusion

Chalking is not calcium carbonate scale, but it appears to calcium oxide bonding with the gel coat for some reason or another. Apart from physically refurbishing the surface, we do not know of a way to clean it up.  Hopefully SC-1000 and good water balance can help you prevent it.  As we learn more information, we will publish it here with an update much like this one.  Thank you.