How To Fix a Hairline Crack in Pebblecrete Pool

How do you fix a hairline crack in a pebblecrete pool?

Two Selleys products are suited to this repair, and both work without draining the pool. Selleys Marine Flex may be suitable and is available in black and white — it can be applied and will cure underwater. The other option for repairing underwater cracks is Selleys Knead It Aqua epoxy repair compound. If the crack is particularly long, contact a pool specialist for advice on the best product to use.

That last point is not a brush-off. A hairline crack in pebblecrete can be a surface-level defect in the render, or it can be the visible end of a structural problem in the shell. The products below fix the first. They will not hold a shell that is still moving.

Choosing between the two

They behave very differently once cured, and the difference decides which one belongs in your crack.

Marine Flex is a flexible adhesive sealant built on Sil-X technology, described by Selleys as giving a more durable seal and bond than traditional polyurethane, with superior UV weather resistance. It stays elastic — hardness 75 Shore A, elongation up to 30% at break. That flexibility is what you want in a joint or a crack that will still see a little seasonal movement.

Knead It Aqua is a hand-kneadable, fast-setting epoxy putty: a white hardener encapsulated in aqua-green resin, which turns off-white when it is mixed properly. Cured, it is extremely hard and inflexible. Selleys states plainly that it is not recommended for filling joints and cracks subject to movement. It is the right choice for a static defect you want to fill and rebuild — and it is the product Selleys names for plugging holes rather than sealing moving joints.

For a hairline crack that may still open and close slightly with water temperature and load, Marine Flex is generally the safer pick. For a small, stable void you want packed solid, Knead It Aqua is.

Published figures for each

Property	Marine Flex	Knead It Aqua
Type	Sil-X adhesive sealant	Two-part epoxy putty
Colours	White, black	Aqua green resin, cures off-white
Pack size	410g cartridge	110g tube; 50g blister
Skin / working time	Approx. 40 minutes	20–30 minutes working time
Cure	Approx. 2mm / 24 hours	Hardens in 20–30 minutes; initial cure 60 minutes; full cure 24 hours at 25°C
Service temperature in water	0–40°C continual; 40–70°C short term	−40 to +120°C continuous
Shrinkage	0%	Good gap-filling; will not sag, run or drip
Underwater use	Applies and cures under water	Can be used under fresh or salt water

These are indicative figures from the products' technical data sheets, not values for preparing specifications.

What those numbers mean in a pool

Marine Flex cures faster when it is wet. Selleys notes shorter skin formation and faster curing in wet conditions and submersion, and longer skin times and slower curing when it is cold. Applying it into a pool in summer is close to the best case for it.

2mm per 24 hours is a depth rate, not a total. A crack you have filled 6mm deep is not fully cured after a day. Selleys' own guidance is to avoid subjecting the repair to water pressure or movement until it is fully cured, so keep the pump off that section and keep people out of it.

Knead It Aqua's 20–30 minute working window shortens in warm water and lengthens below 20°C. Mix only as much as you can place and shape in that time — cut a piece off the roll, knead it between your fingers until the colour is uniform, and press it home while it is still plastic.

Preparing the crack

1. Clean the area so it is free of oil, grease and loose particles. Loose or friable pebblecrete around the crack has to come away — nothing bonds to material that is about to fall off.
2. Where you can, key the surface. Selleys' advice for Knead It Aqua in a hole is to scratch up the inside first so the putty has something to grip. The same logic applies to a crack: a slightly opened, roughened crack holds a repair far better than a smooth hairline one.
3. For Marine Flex, apply with a caulking gun, pushing the sealant into the crack rather than smearing it over the surface, then smooth it off with a spatula within 15 minutes of applying.
4. For Knead It Aqua, force the mixed putty into the crack and feather it out to the edges so the fill is sealed all the way around.
5. Leave it alone. Check critical underwater repairs periodically afterwards and repair again if needed — Selleys recommends exactly that for Marine Flex.

Limitations — where these products should not be used

• Marine Flex cannot be used to repair flexible vinyl pool liners or inflatable pools. This is stated on its technical data sheet. Pebblecrete is fine; a vinyl liner is not.
• Knead It Aqua is not recommended for joints and cracks subject to movement, because the cured product is extremely hard and inflexible. If the crack is opening and closing, it will simply crack again alongside the repair.
• Neither will adhere to polyethylene, polypropylene or Teflon. Relevant if the repair runs up against a plastic fitting.
• Knead It Aqua's set time increases below 20°C. A cold pool in winter will slow it considerably.
• Marine Flex on concrete needs the surface sealed, primed and painted first to be used on its own terms — the technical data sheet lists timber and concrete as substrates for primed and painted use, and it can be applied to fully dried epoxy pool and boat paints. Confirm adhesion with a small sample first.
• A long crack is a job for a pool specialist. Selleys says so directly, and it is the right call — a running crack usually means the shell is moving, and no sealant fixes movement.

Common questions about repairing a pebblecrete pool crack

Do I have to drain the pool?

No. Marine Flex can be applied and will cure under water, and Knead It Aqua can be used under fresh or salt water.

Which colour should I use?

Marine Flex comes in white and black. On light pebblecrete, white is the less visible of the two. Knead It Aqua cures off-white.

How long before the pool can be used again?

Knead It Aqua reaches full cure in 24 hours at 25°C. Marine Flex cures at about 2mm per 24 hours, so it depends on how deep the fill is. In both cases, avoid water pressure or movement on the repair until it has fully cured.

Can I paint over the repair?

Marine Flex is sandable and paintable, and Selleys advises lightly sanding the sealant before painting if the paint does not wet out the surface properly. Knead It Aqua can be machined, drilled, sanded, filed, sawed and painted 60 minutes after mixing.

Will it hold against chlorinated pool water?

Both are specified for underwater use. Knead It Aqua also complies with AS/NZS 4020 for contact with drinking water at temperatures up to 85°C, which is a useful indication of how it behaves in water long term.

What if the crack keeps coming back?

That points to movement in the shell rather than a failure of the product. A repair that reopens in the same place is the sign to get a pool specialist to look at the structure.

Is Knead It Aqua the same as the other Knead It products?

No. Aqua is the variant formulated for damp, wet and underwater substrates. The multipurpose and steel versions are different products with different uses.

What about a gap between the pool coping and adjacent brickwork?

That is a different job — see the gap between brick and swimming pool answer, and sealing a fibreglass pool if your pool is not concrete.