What is polyurea and how does it work?

Polyurea does not come out of a bucket but out of two drums. One holds the resin component, the other the hardener. Kept apart they stay liquid; the reaction only starts when they meet. And they meet at the very last moment: in the head of the spray gun.

That takes a machine that first heats both liquids and then pumps them under high pressure through two separate hoses to the gun. In the mixing chamber of the gun the two jets collide and blend in a fraction of a second. The heat and the pressure keep the mixture thin enough to atomise finely and land on the substrate straight away.

Within seconds of touching the surface the material is no longer liquid. It does not run down a wall or a slope, it does not drip, and it shows no join between two passes. Pass after pass bonds into one closed, rubber-like skin across the whole shell or the whole roof, and within a few hours the surface can be walked on.

That explains why polyurea cannot be applied with a brush, a roller or a simple sprayer. Anyone who mixes the two components in a bucket has a solid block before the brush is in. Only the combination of heating, high pressure and mixing inside the gun itself makes application possible.

No, even though the names look alike. Polyurethane is a relative and is also made from two components, but it reacts far more slowly. That sounds like an advantage, and for a do-it-yourselfer it is one, but a slow reaction has a price: as long as the material is still liquid, plenty can go wrong.

The biggest risk is moisture. A polyurethane that is still curing also reacts with water from the air and from a damp substrate. That produces gas bubbles that stay behind as blisters and pinholes in the layer, and that is exactly where a leak starts later. Polyurea reacts so quickly that moisture in the air barely gets a chance to interfere. That makes application less sensitive to the humidity on the day itself, although rain and wind remain a reason not to spray, and it says nothing about moisture inside the substrate (we come back to that further down).

The second difference is elasticity. Many coatings from a bucket, certainly those based on epoxy or cement, cure hard and rigid. They bond well, but at the first movement of the substrate they break exactly where the stress is greatest. Polyurea stays rubber-like after curing and stretches to several times its own length before it tears.

The third difference is the way the layer is built up. A coating from a bucket goes on in thin coats, often spread over several days, with a join between each coat. Polyurea is sprayed to its full thickness in one working pass and forms a single layer without joins.

Polyurea on its own does not yet make a watertight pool. Our system consists of three layers, each with its own job. Leave one out and sooner or later the whole thing fails.

The first layer is the primer. It is matched to the substrate: concrete, plaster, existing mosaic tiles or polyester each call for a different primer. The primer makes sure the membrane really grips the surface and at the same time acts as a vapour barrier against the normal residual moisture of a dried and measured substrate; no primer helps against a substrate that is still wet.

The second layer is the polyurea membrane, the actual waterproof skin. It is sprayed in one go over the whole shell or the whole roof, including the corners, the edge and the connections of skimmer, lights and drain. There are no seams and no partial areas.

The third layer is an aliphatic top coat in the colour of your choice, matt or satin. It is needed because a bare polyurea membrane changes colour in the sun and turns dull. The aliphatic top coat is UV-stable and holds the colour and the look. Over the years that layer too gradually changes in sheen, which is normal; in many cases the top coat can be renewed after years without replacing the membrane.

Many plots on the Costa Blanca sit on clay. Clay shrinks in a dry summer and swells after the first autumn rain, and a concrete tank weighing tens of tonnes moves with it. Tiles, grout and plaster are rigid. Every movement creates hairline cracks, and through every hairline crack water can escape.

A polyurea membrane moves with the shell. If a new hairline crack forms underneath the membrane, the membrane stretches across it without tearing itself. So the shell is allowed to work a little; the pool stays tight. That is what crack-bridging means in practice, and it is the main reason we work with polyurea here.

There is a limit, though. A crack with a visible step, one that keeps growing measurably or one that points to a sinking edge exceeds the stretch limit of the membrane. Such cracks are first ground open in a V shape, filled or injected and given a reinforcement strip before any spraying is done. If the structure is still moving, a structural engineer should look first. A coating does not hold back a moving foundation.

Polyurea was not invented for swimming pools. The spray technique was developed in the 1980s for industry, where a coating had to cure in a short time and then withstand heavy loads. Hence the properties you now see in your own garden: fast, seamless, tough.

You come across the material as the inner lining of storage tanks and water cisterns, as a protective layer in pick-up truck beds, on parking decks, in wastewater plants and as waterproofing on bridges and roofs. Anywhere water or chemicals must not get through a crack or a seam, and where the substrate moves.

For a homeowner on the Costa Blanca the applications are simpler: the pool shell, the flat roof, the terrace or balcony above a living space, and the water cistern or rainwater tank. In all those cases the principle is the same: one closed skin that moves along.

An honest account of polyurea should also say what it cannot do. First, it is not a structural repair. A cracking or sinking shell does not become stable through a coating; the membrane covers, but it does not carry.

Second, polyurea does not hold back water pressure from outside. A high groundwater level pressing against the back of the shell pushes any coating loose, however well it bonds. That problem calls for drainage, not for a layer on the inside.

Third, it does not protect against moisture from a wet substrate. If spraying is done on plaster that is still wet, that moisture later looks for a way out and ends up as a blister under the membrane. That is why we measure the moisture content before the machine starts, and why the drying time cannot be forced.

This explains why preparation decides the lifespan. Of a renovation of five to ten working days, almost all the time goes into emptying, cleaning, repairing, levelling and priming; the spraying itself takes one day. Whoever saves on preparation saves on the result.

A swimming pool is an unusual environment for a material: it sits under water for months, in water containing chlorine or salt, in full sun, with big differences between day and night. A cured polyurea membrane withstands chlorine and salt water as long as the water values stay within the usual limits. Overdosing or a neglected pH attacks every finish, this one included.

The second advantage is the absence of grout lines. A tiled pool contains hundreds of metres of grout where algae settle and through which water slowly seeps. A closed skin has none of those lines. That saves maintenance and it removes leak points.

The third advantage is that polyurea can in many cases go over existing mosaic tiles. If the tiles are still firmly attached and the substrate is dry and stable, they do not have to come off; loose tiles and loose plaster are removed first. The connections of skimmer, lights, inlets and main drain are taken apart and the coating runs right onto the flange, so there is no seam there either.

With every pool renovation we offer a pressure test of the pipework as standard, carried out by a specialised company. You cannot see a leaking pipe from the shell, and a new skin does not fix it. After the top coat is applied you wait seven days before filling, so that it cures fully. With proper care the expected lifespan of the complete system is 15 to 25 years.

Because polyurea reacts within seconds, there is no second chance. The difference between a pool that stays tight for years and a pool with blisters almost always lies in the person operating the machine and in what they did beforehand. These seven points help you recognise a professional.

Below are the properties from this article in one overview, with what they mean for you as an owner. All the facts are explained in more detail above.

If you have a specific pool, roof or terrace in mind, we take a look on site and you receive a tailored quote within 24 hours of the visit. We work on the Costa Blanca North, from Dénia to Benidorm, and speak English, Spanish, Dutch and German.