A water tank is the one structure where waterproofing works in reverse: you are keeping water in, not out. That single difference changes where the treatment goes, what it has to survive, and what it must not do to the water.
The treatment goes inside
Water pressure inside a full tank pushes outward. A coating on the outside face is on the negative side — the water pressure is trying to lift it off the wall the entire time the tank is full. A coating on the inside face is pressed against the substrate by that same pressure.
So: the inside face is the working face. Treat it, and treat the whole of it — walls, base, and the junction between them, coved, not left square. External treatment is a supplement where the tank is buried and groundwater pushes the other way; it is not a substitute.
What the material has to cope with
- Permanent immersion. Not a rain shower — continuous contact for years. Many perfectly good exterior coatings are simply not formulated for it.
- Hydrostatic pressure. Rising with depth. A deep sump loads the base far harder than a shallow overhead tank does.
- Cleaning. Tanks get scrubbed. The finish has to survive a brush and whatever cleaning agent gets used.
- Filling and emptying. The structure and the coating go through wet-dry cycling constantly, which is harder on a material than staying wet.
The point most people miss: potability
If the tank holds drinking water, the lining is in permanent contact with what your household drinks. Not every waterproofing material is suitable for that, and this is not a detail to assume your way past.
Ask for it in writing. Before anything is applied to a drinking-water tank, get the manufacturer's confirmation that the product is intended for potable water contact, and follow their curing and flushing instructions exactly. We will tell you plainly whether a given material in our range is appropriate for a potable tank, and where it is not, we will say so rather than sell it — ask us before you buy.
After any lining work: fill, stand, drain and flush according to the manufacturer's schedule before the tank returns to use. This is not optional and it is routinely skipped.
Doing the work
- Empty and dry the tank fully. A sump that seeps groundwater may need dewatering held throughout the work.
- Clean back to sound concrete. Remove all algae, scale, laitance and any old failing coating. This is the step that decides the outcome.
- Repair. Cut out cracks, fill honeycombing, make good around every inlet, outlet, overflow and drain pipe. Around penetrations is where tanks leak.
- Cove all internal corners. Wall-to-wall, wall-to-base. Every one.
- Apply as specified — the stated number of coats, in the stated directions, at the stated intervals. Thin coats crossed at right angles beat one thick one.
- Cure properly. Cementitious linings need moist curing and will underperform without it. This is the second most commonly skipped step.
- Test before commissioning. Fill, mark the level, leave 24–48 hours. A drop beyond evaporation means it is still leaking — find it now.
The three cases
Overhead tanks
Shallowest, so lowest pressure, but most exposed to sun on the outside — thermal movement is the stress here, and the roof slab of the tank matters as much as the walls. Access is usually the practical constraint.
Underground sumps
The hardest case, because water pushes both ways. Full tank: pressure outward. Empty tank with a high water table: groundwater pressure inward, which can lift an internal lining off the wall and, in extreme cases, float the empty tank itself. A sump wants a dense, well-built structure with integral resistance, not just a coating hoping to hold the line from one side. If yours is a persistent problem, the honest answer may be that the structure needs attention, not another coat.
Swimming pools
Same principles, higher stakes: continuous immersion, chemically treated water, and a visible finish that must not stain or lift. Specialist territory — treat any advice that flattens pools into "same as a tank" with suspicion.