POOL REPAIRS

What Is Pool Osmosis? Causes, Signs & How to Fix It

Osmosis blistering is one of the most common issues in fibreglass pools across Queensland. If you have noticed small bubbles or blisters forming under the gelcoat surface of your pool, there is a good chance osmosis is the cause. Here is what it is, what causes it, and what it takes to fix it properly.

What Is Pool Osmosis?

Pool osmosis is a process where water gradually passes through the gelcoat surface of a fibreglass pool and becomes trapped between the gelcoat and the underlying fibreglass laminate. Over time, this moisture reacts with chemicals in the resin to form a sticky, acidic liquid. As the liquid builds up, it creates pressure from beneath the surface — forming blisters that eventually crack and damage the gelcoat.

Osmosis is not a manufacturing defect in every case. It can happen to any fibreglass pool over time, especially in warmer climates like South East Queensland where pools hold water year-round.

What Causes Osmosis in Fibreglass Pools?

Several factors contribute to osmosis blistering:

• Age of the pool: Gelcoat naturally breaks down over time. Pools over 10 to 15 years old are most susceptible.
• Poor original gelcoat application: If the original gelcoat was applied too thinly or unevenly, moisture can penetrate more easily.
• Water chemistry imbalances: Consistently low pH, high chlorine levels or poor chemical balance accelerates gelcoat degradation.
• Climate and UV exposure: Queensland's hot climate and intense UV speeds up the breakdown of gelcoat surfaces.
• Voids in the laminate: Air pockets trapped during the original fibreglass layup create weak points where moisture collects.

How to Spot the Signs of Pool Osmosis

Osmosis does not always announce itself obviously. Here are the signs to look for:

• Small blisters or bubbles under the gelcoat, typically ranging from a few millimetres to a couple of centimetres across
• Soft or spongy patches when you press on the pool wall or floor
• A vinegar-like smell when a blister is punctured — this is the acidic liquid trapped beneath the surface
• Discolouration or dark spots that do not respond to chemical treatment or scrubbing
• Cracking or flaking gelcoat around blistered areas, indicating the damage is progressing

If you notice any of these signs, it is worth getting the pool assessed sooner rather than later. Osmosis gets worse over time — the longer it is left, the more extensive the repair.

Not sure whether what you are seeing is osmosis or something else? An independent pool inspection can help identify the issue. Fibreglass Pool Check provides detailed fibreglass pool assessments across South East Queensland.

How Is Pool Osmosis Repaired?

Fixing osmosis properly is not a surface-level patch job. The repair process involves:

1. 1.
   Drain the pool — the pool needs to be fully drained so the affected areas can be accessed and assessed properly.
2. 2.
   Grind back the damaged gelcoat — all blistered and delaminated material is removed down to sound fibreglass. This is the most important step.
3. 3.
   Dry the surface — moisture trapped in the laminate must be allowed to dry completely before new material is applied. Rushing this step is one of the main reasons osmosis repairs fail.
4. 4.
   Apply new fibreglass and resin — fresh fibreglass matting and resin is laid over the prepared surface to rebuild structural integrity.
5. 5.
   Gelcoat topcoat — a quality gelcoat finish is applied to seal and protect the new surface, giving a smooth, waterproof result.

Can You Prevent Pool Osmosis?

You cannot fully prevent osmosis in a fibreglass pool, but you can slow it down significantly:

• Maintain balanced water chemistry — test regularly and keep pH, chlorine and alkalinity within recommended ranges
• Address small blisters early before they spread across the surface
• Have the gelcoat inspected every few years, especially on pools over 10 years old