White Chalky Residue On Fibreglass Pool - What Is It?

White Chalky Residue On Fiberglass Pool - What Is It?

White chalky residue on the walls of your fibreglass pool can be concerning. Sometimes, this condition is a sign of stains developing, while other times it indicates that the protective layer of your pool is oxidising.

In this article, we will talk about the types and causes of chalky residue on fibreglass pools and how you can remove it from your pool.

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What Is White Chalky Residue On Fiberglass Pools?

There are two main types of chalky residue on fibreglass pools: gel coat oxidation and calcium scaling. Calcium scaling is not exclusive to fibreglass pools, but it can sometimes be confused with the signs of gel coat oxidation.

To determine if your pool has calcium scale or gel coat oxidation, observe the behaviour of the chalky stuff when you try to scrub it lightly. If the substance is hard to remove, it is calcium scaling, but if it flakes off fairly easily it is chalking, a symptom of gel coat oxidation.

Causes Of Chalky Residue on Fiberglass Pool

Both types of buildup on fibreglass pools are usually caused by imbalances in your pool’s chemistry.

If your pool’s lining is over twenty years old, it could well be deteriorating on its own, without any issues with the water.

However, many younger pools will still face this problem and in those instances, these symptoms can be corrected by balancing your water chemistry and are not a sign that your pool’s lining needs to be replaced.

Gel Coat Oxidation

Gel coat oxidation that displays as a flaky, chalky substance can be caused by high pH, high total alkalinity levels, high free chlorine levels, or low calcium hardness. Often, this symptom is caused by more than one of these conditions.

Certain amounts of gel coat oxidation are normal, especially if these chemical imbalances are present.

Chalkiness that appears without any imbalances in pool chemistry is rare, but it can be a sign that your gel coating is damaged or ageing. In this case, you may want to consider resurfacing the pool to prevent further degradation of the surface over time.

Calcium Scale

Calcium scale is caused by high calcium hardness in your pool’s water, and can be exaggerated by high pH and high total alkalinity.

When there is a lot of calcium present in your pool water, highly alkaline water can cause this mineral to fall out of solution and deposit on surfaces in the form of white, chalky calcium stains.

How To Remove Chalky Residue From Fiberglass Pool

To remove chalky residue from fibreglass pools, you will need to first balance your pool’s chemistry. Afterwards, you can use specialised products to remove any calcium scaling your pool may have developed.

#1 - Balance pH And Total Alkalinity

Your pool’s pH should be kept at consistent levels ranging from 7.2-7.6. If your pH frequently fluctuates outside of this range, you may have issues with total alkalinity. Ideal levels of total alkalinity in a pool range from 80-120 parts per million (ppm).

If you have high pH and high total alkalinity, you should use a product known as dry acid to lower these levels. You may also choose to use hydrochloric acid for the same purpose, but this product can be more dangerous to handle.

In cases where you need to raise total alkalinity, you should use sodium bicarbonate, also known as baking soda. This product will also create gentle increases in your pH levels. If you need to increase the pH of your pool with less of an impact on total alkalinity, use soda ash.

Read more about the differences between the two.

#2 - Check Free Chlorine Levels

It is common for high free chlorine levels to lead to excess chalking on fibreglass pools due to the corrosive properties of chlorine.

Ideal chlorine levels in pools range from 2-4 ppm, with 4ppm generally considered the upper limit (unless you have very high cyanuric acid). Test your free chlorine levels using a test kit.

Although many pool owners think more chlorine simply means more sanitation, high free chlorine levels can cause severe irritation to your swimmers and create corrosive conditions that can damage more than just your lining.

Balance Calcium Hardness

Although many sources suggest that fibreglass pools require less calcium hardness than concrete or plaster pools, this is not necessarily the case.

We recommend keeping your calcium hardness levels between 200-400 parts per million (ppm) even in a fibreglass pool, as low calcium can lead to hard minerals being leached from your gel coating. This is more likely to happen if there are imbalances in your pH as well.

To raise calcium hardness, use a calcium hardness increaser. Always use small doses when adding calcium to your pool, as this substance is much harder to remove than it is to add.

If you need to lower calcium hardness, you should either perform a partial drain and refill of your pool’s water, or use a calcium reducer.

While these products do not actually remove calcium from your pool, they do prevent it from accumulating on surfaces. The only way to truly remove calcium from your pool is by draining.

Use this calcium hardness calculator to work out how much hardness increaser to add, or how much water to drain and refill to your pool.

Remove Calcium Scaling

To remove calcium scaling from your pool, you will want to use a stain remover. We recommend Serious Stain Remover by Algaefree because this product is safe for use in fibreglass pools and works well on calcium scale as well as other mineral and metal stains.

To use a stain remover, you will typically add the product to your pool with the pump shut off and then leave the mixture to stand for up to three days. After this period has passed, the product will have removed the stains on its own using powerful organic acids.

These stain removers can also tend to lower the calcium content of your pool, so it may not be necessary to use both a stain remover and a calcium reducer.


White, chalky residue in a fibreglass pool is not always a sign of gel coat deterioration. Oxidation is a natural process that occurs over time, and while it is good to take steps to prevent it, the appearance of chalking is often simply calcium scaling, that can be dealt with fairly easily.

Do you have any questions about chalky residue on fibreglass pools? Leave us a comment down below, we’d love to help!

Louis from Pool Advisor


A chemical engineer by trade, Louis is committed to debunking myths in the pool industry by explaining the underlying chemistry and making it accessible to all.