Two fibreglass pools of the same shape can look identical in a catalogue and differ in price by tens of thousands of euros. The difference is almost always in what you will never see once the pool is installed: the number and order of the layers, the type of resin used and the stiffness of the shell. A quality shell has a barrier layer beneath the coloured gelcoat that holds osmosis back — and its presence, or absence, decides whether you will be dealing with blisters under the surface ten years from now.
How a fibreglass shell is actually made
A fibreglass (GRP, composite) pool is not formed by bending sheet material like a metal tub. It is built up layer by layer inside a negative mould — that is, from the inside out. What you see as the glossy surface on the finished pool was the first thing applied during production.
In quality production, the simplified order of layers looks like this:
- The mould — a polished master determines the shape and the surface quality. A worn mould means a wavy surface and poorer gloss.
- Gelcoat — the coloured visible layer, roughly 0.5–0.8 mm thick (Anglo-Saxon sources quote 25–30 mils). It is typically applied in three passes. This is not a paint coating; it is a full-bodied resin layer.
- Barrier coat — a layer of resin with no glass fibres in it. Its purpose is to physically separate the water from the glass reinforcement.
- Vinylester layer with glass reinforcement — the first structural layer, still made with vinylester resin because of its resistance to water.
- Structural glass layers — several layers of chopped strand mat or roving (commonly 375–450 g/m²) with resin. This is where the strength comes from.
- Core and stiffeners — better shells use a core material (ceramic microspheres, honeycomb core, closed-cell foam) and ribs at the most highly stressed points: the top edge, the wall-to-floor transitions, the steps and the seats.
Cheap shells skip some of these steps. The barrier coat and the vinylester resin are the ones most often missing — the whole laminate is then made from cheaper polyester. The pool looks the same for the first season. The difference only shows up years later.
What osmosis is and why pool people fear it
Osmosis is the process by which water molecules gradually penetrate the gelcoat and enter the laminate. Gelcoat is not an absolute barrier — it is a semi-permeable layer. Water finds its way to the micro-voids left in the laminate during production, or to residues of incompletely cured resin.
Inside, hydrolysis then takes place: water reacts with the ester bonds in the resin and produces water-soluble, acidic products. These draw in more water by osmotic pressure. The pressure inside the void rises and the gelcoat above it bulges into a blister.
In practice it looks like this: after five to fifteen years, small blisters ranging from lentil-sized to coin-sized appear on the walls or the floor. At first you can only feel them; later they burst and expose a dull layer underneath. Cut one open and a sour, vinegar-smelling liquid runs out. The pool is not leaking and still holds water — this is primarily a cosmetic, surface defect, but over time it degrades the laminate below the surface.
What slows osmosis down
The type of resin. Vinylester resin has substantially fewer vulnerable ester bonds and better elongation than ordinary orthophthalic polyester. The boatbuilding industry, which has been fighting blisters for decades longer than the pool trade, therefore specifies a vinylester skin coat beneath the gelcoat as standard. The same applies to pools: vinylester in the first layers is the most effective prevention a manufacturer can build into a shell.
A glass-free barrier coat. A resin layer containing no glass fibres makes it practically impossible for water to reach the glass directly. The fibre itself acts as a wick — wherever it is poorly wetted out with resin, it carries moisture further into the structure.
A core that does not absorb water. Ceramic microspheres in a vinylester matrix, closed-cell foam or honeycomb — all have one thing in common: they are non-absorbent. Cheap fillers based on ground limestone, or an open structure, hold moisture instead.
Production quality. Even the best formulation will not help if the laminate is not properly de-aerated, if it cures in a cold workshop, or if the layers are applied outside the prescribed time window. Air bubbles in the laminate are the starter voids for future blisters.
It is also worth noting that osmotic blisters do not only form from the inside. If the pool is set into wet ground with no drainage, moisture can act from the outside as well — which is why drainage of the excavation is part of a quality installation, not an optional extra.
Gelcoat: the four parameters that matter
Thickness. Below 0.4 mm the surface is easily sanded through during ordinary cleaning and leaves no reserve for polishing. Quality manufacturers stay around 0.6 mm and measure the thickness at several points in the mould, not just on the floor.
UV stability and colour fastness. Pigments and UV stabilisers are what decide here. Dark and deep blue shades are the most demanding — with cheap gelcoats, chalking shows up after a few seasons (the surface goes matt and leaves powder on your hand when you wipe it) or the colour fades unevenly above the waterline, where exposure is greatest.
Chemical resistance. Gelcoat lives permanently in an environment with active chlorine and fluctuating pH. Persistently low pH or direct contact with undissolved chlorine granules will dull and etch the surface — that is not a product defect but an operating one. We covered this in more detail in our article on water hardness and scale buildup.
Repairability. This is the key practical advantage of gelcoat over liner: a scratch can be sanded back and refinished. A gelcoat kit in the pool’s colour, abrasive paper in progressive grits and polishing compound are enough for minor defects. That is why it pays to ask for the colour code when you buy — ten years later the manufacturer may not be able to trace it.
Shell stiffness: why a thin shell “works”
A fibreglass pool is a flexible shell. On its own it cannot take the lateral pressure of the soil — that is carried by the backfill or the surrounding concrete. As long as the pool is full of water, the pressures largely balance out. Problems arise when it is drained, when it is filled without backfilling at the same time, or when it is mishandled by the crane.
A thin shell without adequate stiffeners deforms at exactly these moments: the top edge ripples, the walls bulge inwards, a wave forms at the steps. If this happens during concreting, the concrete locks the deformation in permanently. And because the laminate is flexible, repeated flexing in one spot leads to micro-cracks in the gelcoat — so-called hairline cracks, which are the entry point for water into the laminate.
The hallmarks of a stiff shell are a substantial top rim (often with a closed profile or a core), transverse ribs on the floor of larger sizes, and a noticeably greater wall thickness at the floor than at the edge. On a finished pool you can gauge stiffness easily: press the palm of your hand against the wall of an empty shell at about two-thirds height. A quality shell barely moves at all.
The matching procedure on site, including the most common concreting mistakes, is covered in a separate article on backfilling and concreting a pool shell. For a comparison of fibreglass with other constructions, see our overview of pool construction types.
Table: quality feature → what it means → how to verify it
| Quality feature | What it means | How to verify it |
|---|---|---|
| Vinylester resin in the first layers | Substantially higher resistance to hydrolysis and osmosis than an all-polyester laminate | Ask for the technical data sheet with the layer build-up; a vague “quality resin” is not enough |
| Barrier coat with no glass fibres | Water cannot reach the glass directly, which would otherwise act as a wick | The data sheet must list it as a separate layer between the gelcoat and the laminate |
| Non-absorbent core (ceramic microspheres, honeycomb, closed-cell foam) | Stiffness with no added weight and no internal moisture | Ask what the core material is and exactly where in the structure it sits |
| Gelcoat of roughly 0.5–0.8 mm | Reserve for polishing, scratch repairs and long-term colour fastness | Ask for a measured figure from production, ideally from several points in the mould |
| Wall thickness varying with load | The material is where it is needed — more at the floor and in the corners than at the edge | Ask to be shown a section through the shell or an offcut from production |
| Substantial top rim and ribs | The shell does not move during transport, setting and concreting | Press on the wall of an empty shell; a quality shell will not budge |
| References older than 10 years | The only real-world test for osmosis and UV stability that exists | Ask for the contact details of an owner from an installation done in 2010–2015 |
| Separate warranties for the shell and the surface | An honest manufacturer distinguishes structure from cosmetics; a single figure for everything is a warning sign | Read the warranty terms before you sign, not after delivery |
Checklist: what to ask the dealer
Frame the questions so that they cannot be answered in one word. A dealer who knows the product will answer without hesitation; one who is selling an unknown imported shell will start talking in generalities.
- What is the exact layer build-up, from the gelcoat outwards? You want to hear the order and the purpose of each layer, not a marketing name.
- What type of resin is used in each layer? Vinylester, isophthalic polyester, orthophthalic polyester — and exactly where.
- What is the shell thickness at the floor, at the wall and at the top edge? A single figure for the whole pool means nobody measured it.
- Is there a core in the structure? Made of what, and where? And does that material absorb water?
- How many years’ warranty is there on the shell and how many on the surface — and what exactly does each cover? What matters most is whether osmosis is named specifically among the covered defects, or among the exclusions.
- Is the warranty transferable to a subsequent owner? It affects the value of the property when you sell.
- Can I see an installation more than ten years old? Ideally one the dealer did not pick out for you.
- Who will honour the warranty if the manufacturer goes out of business? With imported shells that have no local representation, this is a real risk.
- Will I get the gelcoat colour code and a recommended repair kit? Without them you will not be repairing anything ten years from now.
What to do when the problem has already occurred and the dealer is lukewarm about it is summarised in our article on warranty claims for pools and hot tubs — including how to document the defect so that the claim stands up.
Damaged gelcoat: repair or refurbish?
The dividing line is fairly clear, and it is more about the extent than the depth of the damage.
Spot repair makes sense for individual scratches, tool marks and isolated blisters. The procedure: open the blister, grind the void back to sound laminate, wash it out thoroughly and let it dry completely (weeks, not days — residual moisture is the most common reason a repair pops back up a year later), fill with resin, cover with gelcoat and polish. The cost runs to a few hundred euros.
A full refurbishment is called for when the blistering is widespread, the gelcoat is matt and chalky across the whole surface, or there are hairline cracks along the walls. Here you are looking at sanding the entire surface back and applying new gelcoat, spraying on a polyurea or epoxy system, or lining with a liner. The choice depends on the condition of the laminate underneath — and that has to be assessed by someone in person, not from photographs. We go through the options and indicative costs in our article on renovating an old pool.
One rule applies to both options: never repair gelcoat over wet laminate, and never start until you know where the water is getting into the structure. Otherwise you are repairing the symptom while the cause carries on working.
Frequently asked questions
Does one blister mean my pool is defective?
Not necessarily. An isolated void can be the result of a local lamination fault and can be spot-repaired. What is worrying is when blisters keep appearing, come up in clusters and have that characteristic sour content — that points to a systemic problem with the layer build-up rather than to chance.
How long does osmosis take to show up?
In a pool with no barrier coat, typically after five to fifteen years of operation, and sooner in pools kept permanently full at a higher water temperature. That is exactly why references ten years old and more are the only reliable test — a five-year-old installation tells you nothing about osmosis.
Does gelcoat on its own protect against osmosis?
Not sufficiently. Gelcoat is semi-permeable and its main job is appearance, UV stability and the chemical resistance of the surface. What actually works against water penetrating the laminate is the combination of gelcoat, a glass-free barrier coat and vinylester resin in the first structural layers.
Is a more expensive fibreglass pool always better?
Price on its own proves nothing — part of the difference goes on shape, equipment and margin. But the layer build-up described above costs money: vinylester is substantially more expensive than ordinary polyester, and so are core materials. An extremely cheap shell almost certainly does not contain these layers. So ask about the build-up, not about the price.
Can barrier protection be added afterwards?
Not from the outside, and only to a limited extent from the inside. There are epoxy systems that are applied to dried and sanded laminate and work as an additional barrier. But that is a refurbishment measure costing thousands of euros, not prevention. The decision about shell quality is made when you buy, not afterwards.
In closing
A buyer choosing a fibreglass pool has neither a microscope nor a laboratory at their disposal — but they do have a right to a technical data sheet and to references. Ask about the layer build-up, the type of resin, the thicknesses at individual points and what exactly the surface warranty covers. A manufacturer with something to show will answer within a minute. One who dodges the question is answering too — just differently.
If you are only just starting to choose, browse the range in the pools category or work through the other texts in the Guide, where we go into individual constructions, technologies and running costs in detail.
