How do you make a swimming pool permanently waterproof?

Glass mosaic tiling is the classic finish here. The tile lets no water through, but the tile is not the waterproofing: the grout and the adhesive bed behind it are. Grout shrinks and washes out, and leaks nearly always start at the cove between wall and floor, in the corners and around fittings. A cement-based render is cheaper and looks crisp, but it is rigid: when the shell moves it cracks, and those hairline cracks run down to the concrete.

A fitted membrane or liner leaves the factory watertight but is welded and clamped on site, with collars around the skimmer, the inlets and the lights. Every weld and every collar can fail. Pool paint is a finish rather than a waterproofing layer: too thin to bridge cracks, and it peels as soon as moisture pushes against it from behind. The sprayed membrane has no joints in the field, so its weak point lies elsewhere, in the connection details and in the preparation.

A pool shell is never still. On the Costa Blanca, temperatures swing widely between day and night and between summer and winter, and the ground is dry and prone to movement. Concrete expands and contracts again, the subsoil settles, and that movement ends up in the weakest link: the joint.

Water pressure adds to it, bearing on every square centimetre day and night and constantly seeking the smallest opening. Chlorine, salt and swinging pH values attack grout and sealant edges faster than they attack the tile. And every joint is a separate transition with a lifespan of its own: the finish does not fail as a whole, it fails at the spot that is first due for renewal.

The pool has to be empty and dry. Loose and old layers are removed and any tiling that sounds hollow is taken off, because whatever is already coming away from the concrete will take the new layer with it later. Cracks are routed out until they are wide enough to fill properly, then closed. Where needed, a levelling layer of cement-based mortar is applied, roughly 15 kilos per square metre for a layer one centimetre thick. That layer has to cure, and the waiting is part of the job; finally the whole shell is made dust free.

Residual moisture and an insufficiently cured substrate are the most underestimated cause of failure. A coating seals the shell, so moisture still inside has only one way to go and pushes against the back of the new layer. The result is blistering or osmosis, sometimes only visible months later.

On the dry, prepared substrate comes a bonding primer suitable for permanent immersion, in two coats totalling roughly 500 grams per square metre; that primer also acts as a vapour barrier. The polyurea membrane is then sprayed on, roughly 2 kilos per square metre with a final thickness of at least 2 millimetres, in one continuous pass across floor, walls, corners and steps. Over that goes the UV and wear layer, rolled on in two coats totalling roughly 400 grams per square metre, the only layer that can handle permanent immersion.

The spraying itself is a single day's work on an average pool. Preparation takes by far the most time, which is why a complete renovation averages 5 to 10 working days. The pool then has to be left alone for seven days before it is filled, so that the top layer cures fully.

The flat area of a pool is the easy part. The work sits in the fittings: skimmer, inlets, main drain and lights. At each of those the continuous layer stops and a connection to another material begins. The same applies to the transition to the coping and, on an overflow pool, to the channel, where water runs across permanently.

Movement joints need their own treatment: they are there for a reason and must not simply be filled over, because the movement then transfers to the section beside them. A system is only as strong as its worst connection; a perfectly sprayed field with one carelessly finished fitting is not a watertight pool.

With a new shell, one rule outranks all others: the concrete has to cure and dry sufficiently first. Fresh concrete holds a great deal of water and gives it off for months, even when the surface feels dry.

Coat too early and that moisture is locked in. Sun on the outside raises the vapour pressure inside the structure, and that pressure looks for a way out, straight against the new layer; blistering and local loss of adhesion follow. With an existing shell the question is what is on there now, how well it is bonded and what has to come off.

Keep the water balance and the pH in order, because overdosing chlorine or salt and leaving values out of range for long periods puts a strain on any finish. Do not use aggressive or unknown products without knowing whether they are compatible, and do not clean at pressures above the recommended values. Do not leave the pool empty for long either: with a high water table an empty pool can float.

We give ten years of guarantee on watertightness and adhesion, five years on the colour stability of the top layer and three years on the finishing details, counted from the date of completion. The service life we expect is 15 to 25 years. That is an expectation based on the material, not a guarantee.

A coating is a skin, not a structure. If the shell is failing structurally, that has to be resolved as building work first; a layer over the top hides it for a season at best. Where there is water pressure from outside or a high water table, the water pushes against the back of the layer, and sealing from the inside is not the right answer.

The same goes for a pool that is still settling: waiting is wiser than finishing. If the substrate has been prepared by others and we cannot verify the build-up underneath, we cannot stand behind the result. And on a small above-ground pool the effort is out of proportion to what it delivers. We would rather say so beforehand than afterwards.

Whether waterproofing in your case means repairing, refinishing or fully renewing can usually be judged quickly from the edge of the pool. We come out for that without obligation in the province of Alicante, and we can also give a first impression from photographs.

Useful photographs are of the whole shell, of the cove between wall and floor and of the fittings, together with the approximate dimensions. After the inspection you receive a quotation within 24 hours setting out the build-up, the colour and the expected duration.