Polyurea or a liner for my pool: what is the difference and which should you choose?

For a liner, the shell is first made smooth, because every irregularity will show through the sheet. The material is then cut to the measured shape, the panels are welded at the seams and the whole sheet is hung in the pool, pulled tight and fixed into the edge profiles, after which the inlets and the skimmer are cut through and sealed.

With the sprayed system, most of the time goes into preparation: removing old layers, repairing cracks, cleaning and levelling the surface, where necessary with a cement-based levelling mortar at roughly 15 kilos per square metre for a layer of one centimetre. Next come two coats of bonding primer for permanent immersion, together about 500 grams per square metre, which also acts as a vapour barrier. The polyurea membrane is sprayed in one continuous seamless layer of around 2 kilos per square metre, with a final thickness of at least 2 millimetres. Over that goes the UV and wear layer, rolled on in two coats, together about 400 grams per square metre.

A liner is usually in place within one or two days and the pool can be filled again soon afterwards. A full renovation with the sprayed system takes five to ten working days on average; the spraying itself is a single day's work on an average pool. Filling may only start after seven days, because the top layer needs that time to cure.

A liner is flat material forced into a three-dimensional shape. In a rectangular pool with a straight staircase that works out neatly. With Roman steps, a seating bench, a niche for the lighting, a recess at the overflow channel or a curved free form, the sheet has to put its surplus material somewhere, and that becomes a fold or an extra welded seam. Both are weak points: a seam can let go and a fold holds dirt.

The spray gun follows the shell as it is, over the nose of every step and into the inside corner of a niche, delivering the same continuous layer everywhere. The more irregular your pool, the larger that difference turns out to be.

A liner is usually replaced after roughly 8 to 12 years. That is not a fault but the normal course of things: the sheet fades, becomes stiffer and the seams and edge profiles wear out. So do not count on a single job, but on one that repeats, each time with draining, dismantling, fitting and recommissioning.

On the sprayed system we give a ten year guarantee on watertightness and ten years on adhesion. The expected service life is 15 to 25 years. That is an expectation and explicitly not a guarantee. Counted in replacement cycles, that is where the difference sits: within the period a coated pool is expected to still be doing its job, a liner will as a rule have been renewed more than once.

Sheet material is vulnerable to anything sharp or heavy. A parasol base tipping over, a small stone under a shoe or a dog hauling itself out of the water with its claws are the classic causes. A small leak can be patched underwater and often holds for years, but the patch stays visible. If the damage sits on a seam or in a fold, patching is rarely permanent.

The sprayed membrane is tougher and has no seams along which damage can spread. Where mechanical damage does occur, the spot is abraded, cleaned and made good with the same system, so that there is a single whole again. Just afterwards you can still see a slight difference in gloss; after a season of sun and water it is hard to spot. With both systems, mechanical damage falls outside the guarantee.

Both leave you free on colour: liners come in set colours and patterns, and with the sprayed top layer almost every RAL colour is possible, because the pigment runs through that top layer itself. Underfoot a liner feels softer; the sprayed layer feels firmer, like a smooth wall that warms up quickly in the sun.

Polyurea resists chlorine, salt water and pool chemicals while staying elastic. With a liner the sensitive point lies in the seams, the folds and the connections: that is where biofilm takes hold most easily, where the dark staining appears and where a brush reaches worst. On a seamless layer, brush and robot simply run over one continuous surface.

On the Costa Blanca the sun is added to all this. Fierce light, high UV exposure and large temperature swings make a shell here work harder than one in northern Europe. A liner fades noticeably as a result, above all at the waterline, while the membrane stays elastic and bridges movement in the substrate.

A liner hangs inside the shell and hides what is behind it: an old, stained wall with the odd crack disappears from view. A coating does the opposite, because it becomes part of the shell and therefore follows everything that is not in order there. That is why the substrate has to be repaired, cleaned, levelled and dry enough first.

Sometimes that argues for the liner: if the shell is only cosmetically poor, you are quickly done. It also argues against the liner, because a problem simply carries on underneath the sheet. Water pressing against the shell from outside, or a crack that is still moving, will make itself known later, with the difference that you no longer see it coming.

The most common mistake is to compare the two on the purchase alone and leave out the rest: the condition of the shell, the shape of the pool and how many times you want to repeat the job. The second mistake is coating a substrate that is not ready for it; residual moisture, a crack that is still moving or an expansion joint treated incorrectly will cause trouble later and fall outside the guarantee. From the other side there is the misconception that a liner needs no upkeep: edge profiles, sealant and the connections at the skimmer and inlets let the water go first, long before the sheet itself is finished.

If the budget is limited and that really is the line, choose the liner; a properly fitted liner beats a half-executed coating. If you are selling the property before long, the longer lifespan will not come back to you. For a holiday home in light use, that difference also weighs less heavily.

If the pool is to be enlarged, reshaped or replaced within a few years, do not choose a system built for a service life of 15 to 25 years. And if the shell is still settling, for instance in new build or after groundwork nearby, waiting is wiser than pressing on; a liner bridges that period until the structure comes to rest. We would rather say so beforehand than afterwards.

Which of the two suits your pool usually becomes clear quickly once someone looks at the condition of the shell, the shape, the way it is used and the years you still want out of it. That inspection is free of charge and can also be done from photographs; the quotation follows within 24 hours. If the conclusion is that a liner makes the most sense, we will simply say so.