Fibreglass pool problems: osmosis blisters, gelcoat wear and cracks

How a fibreglass shell is built

A fibreglass pool is moulded from the inside out. River Pools, a US manufacturer, explains the order: the gelcoat goes into the mould first and becomes the coloured surface you see; behind it comes a resin barrier layer (vinyl ester in better shells), and then the structural laminate of glass fibre and resin. Most problems can be traced to one of those layers.

The same structure explains why a fibreglass pool behaves differently from concrete when it is drained, which matters before any repair (see below).

Osmosis blisters

What they look like. River Pools describes osmotic blisters as raised bubbles on the surface, normally the size of a small coin or smaller. They do not go away by themselves.

Why they form. Gelcoat is not completely waterproof, and water molecules pass through it slowly. If they reach a resin that reacts with water, they combine with compounds in the laminate into larger molecules that cannot pass back out, and the gelcoat lifts. River Pools says the vinyl ester layer is there to stop water reaching the cheaper polyester resin behind it, and that blisters were not uncommon before vinyl ester became the industry standard.

Hitchins Technologies, an Australian maker of pool coatings, describes the same process as hydrolytic degradation of permanently immersed laminates and links it to how the shell was made: poor wetting of the fibres, trapped air and the quality of the resin. In its experience osmosis tends to show 10 to 15 years after construction, although it can appear sooner or later.

How far it has gone. Hitchins describes a range. A few isolated blisters may be the whole problem, or the first sign of something larger. It suggests repairing isolated blisters and then monitoring, sometimes for two or three years, before deciding on a long-term strategy. At the far end of the range, the answer is removing the gelcoat and installing a new fibreglass layer.

How a local repair works. For isolated blisters, Hitchins publishes this method:

1. Empty the pool (with care; see the section on draining).
2. Grind out each blister and the area around it, to a shallow depth.
3. Dry the area thoroughly, which can take several dry days.
4. Fill with glass cloth and epoxy, then sand flush with the surrounding surface.
5. Apply the finishing coat.

Hitchins adds that blisters are easiest to see straight after the pool is emptied, because they deflate over time. Ask the specialist to inspect as soon as the water is down.

Fading, chalking and staining

Thursday Pools, another fibreglass manufacturer, links chalking, scale and discolouration to water chemistry. It recommends pH between 7.2 and 7.4, free chlorine between 1 and 3 ppm and calcium hardness below 120 ppm, and warns that high calcium hardness combined with high pH can leave white deposits or rough scale on the gelcoat. Calcium-based chlorine products add calcium to the water.

Its advice is to identify the cause before treating a stain, because staining does not necessarily mean the gelcoat is damaged and different stains need different treatments. Avoid wire brushes, abrasive pads and metal scrapers, which scratch the surface. Use soft cloths, nylon brushes and products approved for fibreglass.

Some surface faults come from manufacture. River Pools explains that gelcoat applied too thin can show white spots, and that too much or too little catalyst can shorten its life, crack it or turn the surface white.

Cracks

Spider or hairline cracks. Thursday Pools says these are usually limited to the cosmetic gelcoat and do not affect the structure. River Pools adds that gelcoat applied too thick becomes rigid and can crack during shipping or lifting. A fibreglass specialist should confirm whether a crack stops at the gelcoat or continues into the laminate. Thursday Pools also notes that not every cosmetic crack needs repairing, because the repair can be more visible than the original hairline.

Structural cracks and bulging walls. Thursday Pools attributes these to installation errors, unsuitable backfill, soil movement and groundwater pressure. When groundwater outside the shell rises higher than the water inside, it pushes the wall inwards. The repair has to deal with the cause (support or water pressure) as well as the visible damage.

Draining a fibreglass pool

Do not drain a fibreglass pool yourself. Thursday Pools explains that the water inside counters groundwater pressure from outside, and removing it can let the shell shift, crack or lift. It also says lowering the water below the skimmer can void the pool warranty. Any repair that needs the pool empty should be planned by someone who has assessed the ground and groundwater on your site.

Tiles on fibreglass

Many fibreglass pools have a tiled band at the waterline. ARDEX UK notes that traditional cement-based adhesives cannot bond to fibreglass (GRP) shells and that it developed a polyurethane reaction resin adhesive for the purpose. If waterline tiles on a fibreglass pool keep coming loose, ask what adhesive was used.

When to call a specialist

• new blisters, or blisters that are spreading
• a crack you can catch with a fingernail, or one that is growing
• walls that look bowed or out of line
• tiles lifting at the waterline
• any plan that involves draining the pool

Photographs with a coin for scale, and the age of the pool, help a fibreglass repairer judge whether a local repair or resurfacing is the likely answer.