Fibreglass pool osmotic blistering: causes, cost, and what to do

Osmotic blistering is the most common serious problem I see on fibreglass pools. It looks minor — small raised bumps on the pool surface — but it's a sign that water is penetrating the gelcoat and getting into the structural laminate underneath. Left unchecked, it leads to delamination, and that's when repair costs escalate.

This article explains what osmotic blistering actually is, why it happens, what it costs to fix, and how to tell whether your pool needs urgent action or can be monitored.

What osmotic blistering looks like

Osmotic blisters appear as small, raised bumps on the pool surface — usually below the waterline. They can be as small as a few millimetres or as large as a 50-cent coin. In early stages, you might only notice them when the light hits the wall at a certain angle. In advanced cases, the surface looks visibly bumpy and uneven.

If you press a blister and it feels soft or spongy, that's a sign it's filled with fluid — a mix of water and dissolved chemicals from the laminate. If you break a blister open (which I don't recommend doing without professional assessment), you'll often find a dark, vinegar-smelling liquid inside. That liquid is acidic and is actively breaking down the fibreglass beneath the surface.

What causes osmotic blistering

Osmosis is a natural process where water moves through a semi-permeable membrane (in this case, the gelcoat) toward a higher concentration of dissolved substances (the chemicals in the fibreglass resin).

In plain terms: pool water slowly seeps through microscopic pores or imperfections in the gelcoat and reaches the resin and fibreglass layers beneath. Once inside, it reacts with the resin and creates small pockets of pressurised fluid. Those pockets push the gelcoat outward, forming blisters.

Several factors influence how quickly this happens: the quality of the original gelcoat application, the resin type used in manufacturing, water chemistry (especially pH imbalance), water temperature, and the age of the pool. Some pools develop blisters within 5 years. Others go 20+ years without a single one. It depends heavily on the quality of the original manufacturing process.

Is it always serious?

Not always — but it always needs attention.

A small number of scattered blisters on an older pool (15+ years) might be cosmetic-level. They can sometimes be individually repaired without a full resurface, especially if the underlying laminate is still solid.

Widespread blistering — dozens of blisters across large areas of the pool — is a different story. That indicates the osmotic process is well underway and the gelcoat barrier has failed across a significant area. At that point, individual blister repair is a waste of money. The pool needs a full resurface: stripping the old gelcoat, treating the laminate, and applying a new surface system.

The biggest risk with osmotic blistering isn't the blisters themselves — it's delamination. If water penetrates deep enough into the laminate, the fibreglass layers start to separate from each other. Delamination weakens the structural integrity of the pool shell and is significantly more expensive to repair than surface blistering alone.

What does it cost to fix?

Repair costs depend on the severity and the size of the pool. Here are typical ranges I see in Australia:

These are indicative ranges based on pools I've assessed and resurfaced. Actual costs vary by pool size, location, the coating system used, and the contractor. Which is exactly why I recommend getting quotes reviewed before you commit — the difference between contractors can be thousands of dollars for the same pool.

How to tell if you need a full resurface

This is the question I get asked most often. Here are the indicators I use when assessing a pool:

If you're unsure which category your pool falls into, that's exactly what the Pool Check Report is for. Send me photos and I'll give you a straight answer.

What to do if you're buying a property with a blistered pool

If you notice blistering during a property inspection, don't panic — but don't ignore it either. Get an independent assessment of the severity and likely repair cost. That information gives you three options: renegotiate the purchase price to account for the repair cost, request the vendor fix it before settlement, or walk away if the numbers don't work.

A $149 pool assessment that identifies a $15,000 problem is one of the best returns on investment you'll ever get in property.

Can you prevent osmotic blistering?

You can reduce the risk, but you can't eliminate it entirely — because the primary factor is how the pool was manufactured, which you can't change after the fact.

What you can control is water chemistry. Keeping pH balanced (7.2–7.6), avoiding prolonged low pH conditions, and maintaining proper sanitiser levels all help protect the gelcoat. High water temperatures accelerate osmosis, so pools in tropical climates tend to develop blisters sooner than pools in cooler regions.

If your pool has been resurfaced with a modern epoxy or vinyl ester barrier coat system, the risk of future osmotic blistering is significantly reduced. The barrier coat acts as a much more effective water barrier than the original gelcoat.

Got a resurfacing quote? Get it reviewed.

If you've already been told your pool needs resurfacing and you have a quote, the Quote Review ($79) is worth doing before you sign. I'll check whether the recommended scope of work is appropriate, the materials are right, and the price is within the expected range.

I've seen quotes that underspec the surface system to win on price (which leads to the problem recurring within a few years), and I've seen quotes that include unnecessary work to inflate the total. An independent second opinion costs $79 and can save you thousands.