Sealing Poly Rainwater Tank and Plastic Storm Pipe

Which sealant suits a 15mm gap between a poly rainwater tank and a plastic stormwater pipe?

Unfortunately, there is no Selleys sealant suitable for this application. Polyethylene plastic is very difficult to adhere to, and a poly tank is polyethylene by definition.

That answer holds even though every other requirement in the question is one Selleys products would normally handle well — a 15mm gap, an outdoor exposure, and a joint that has to stay flexible because the pipe is attached to a 12-metre shed wall that will expand and contract. It is the material, not the job, that rules the sealants out.

Why polyethylene defeats sealants

Polyethylene has very low surface energy. An adhesive or sealant applied to it beads up rather than wetting out across the surface, so it never achieves the intimate contact a bond needs. It is also chemically inert, so there is nothing for a sealant to react with.

This is not a gap in the Selleys range — it is a property of the material, and it is stated on technical data sheet after technical data sheet:

• Roof & Gutter Silicone — not for use on polyethylene, polypropylene or PTFE (Teflon).
• Brick & Concrete Silicone — not for use on polyethylene, polypropylene or Teflon; pre-test plastics before use.
• No Mould and Wet Area Silicone — not suitable for PE, PP or PTFE.
• Flexiseal FC — not suitable for PE, PP, PTFE and some plastics; test first.
• Knead It Aqua — will not adhere to polyethylene, polypropylene or Teflon.

The one product that will bond these plastics is Selleys All Plastics, a cyanoacrylate with a surface-activating primer pen that achieves 5.6 MPa on polyethylene and 2.9 MPa on polypropylene. It is a 3g tube intended for repairing toys, games, office items, remote controls and ornaments, and it cures rigid. It is not a sealant, there is nowhere near enough of it for a 15mm joint, and a rigid bond is the opposite of what a flexing joint needs.

The second problem: this joint has to move

Even if adhesion were solved, this particular joint is demanding. A 90mm pipe entering a tank through a hole cut in the corrugations, with the pipe fixed along a 12-metre shed wall, will move relative to the tank as the wall expands and contracts in the sun. Twelve metres of shed wall moves a surprising amount over a summer day.

A sealant bead across that gap would be asked to absorb that movement continuously, on a substrate it cannot grip. It would fail — the only question is how quickly.

What actually works for this connection

Plumbing and tank systems solve this mechanically rather than chemically, and the components are readily available:

• A proper tank inlet fitting or bulkhead fitting. These clamp through the tank wall and seal on a gasket compressed by a nut, not on adhesion.
• A flexible coupling between the pipe and the tank inlet, which absorbs the differential movement by design.
• A rubber grommet or boot sized to the pipe, seated into a correctly sized hole.
• An expansion joint in the pipe run itself, so the movement is taken up somewhere other than at the tank.

Your tank manufacturer will specify the inlet fitting for that tank, and a plumbing supplier or irrigation supplier will have the flexible couplings. Both are the correct answer here in a way no sealant is.

A note on the corrugations

There is a further practical difficulty worth naming. A hole cut through corrugated tank wall does not present a flat face for anything to seal against — the profile means the gap around a round pipe varies continuously as it crosses each corrugation. Even a sealant that could bond to polyethylene would be trying to bridge a gap that is 2mm in one place and 15mm in the next.

Mechanical fittings for corrugated tanks are made with flanges and gaskets designed to bridge that profile. That is the specific problem they exist to solve.

Safety: what the one product that does bond polyethylene carries

Only one product named above will grip a poly tank, and it is also the only one on this page with a serious classification. Selleys All Plastics is classified hazardous with the signal word Danger: H225 (highly flammable liquid and vapour), H319 (causes serious eye irritation) and H336 (may cause drowsiness or dizziness).

Those classifications belong to the primer pen, not the glue. The primer is over 60% isopropanol, and it is the half of the system that makes polyethylene bondable at all — which means you cannot use the product on a poly tank without using the flammable component. Category 2 flammable means the vapour forms an ignitable mixture at ordinary air temperature, not at some temperature you would have to reach.

What that means for this particular job. The product's own guidance is that its vapour is heavier than air, collects in hollows, and that you should not work in a confined space. A tank inlet is usually at the base of a tank tucked against a shed wall, often in a narrow, unventilated run between the tank and the cladding, and frequently near a pump. A pump motor is an ignition source at exactly the height the vapour sits. If you are going to test the primer on an offcut — and you should test rather than commit — do it in the open, away from the pump, with the pump isolated.

The H319 matters because of the geometry: a 3 g tube and a pen are used at close range, at arm's length or less, often looking up at an inlet. Chemical goggles or safety glasses, nitrile gloves, and keep the pen capped when you are not using it.

The lower-hazard option is the answer this page already gives. A bulkhead or tank inlet fitting with a compressed gasket, a flexible coupling and a rubber grommet carry no chemical classification at all. They are the correct fix for a 15 mm moving joint on polyethylene, and choosing them removes the flammable product from the job rather than managing it.

Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand.

Common questions about sealing a poly tank connection

Is there any Selleys product that will stick to a poly tank?

Selleys All Plastics will bond polyethylene with its primer pen, but it is a 3g rigid cyanoacrylate for small repairs, not a sealant for a 15mm flexible joint.

What about an epoxy putty?

Knead It Aqua will not adhere to polyethylene or polypropylene, and its own data sheet says it is not recommended for filling joints and cracks subject to movement because the cured product is extremely hard and inflexible. Both exclusions apply here.

Would roughing the surface up help?

Sanding improves mechanical key on many plastics, but it does not change polyethylene's low surface energy, which is the actual barrier. Chemical priming is what All Plastics uses to get around it.

How can I confirm my tank is polyethylene?

Most poly tanks carry a material marking. If it is a moulded plastic rainwater tank, polyethylene is the overwhelmingly common material.

Is the stormwater pipe the same problem?

Not necessarily — stormwater pipe is often PVC, which is a much easier substrate. Roof & Gutter Silicone lists PVC among its suitable non-porous substrates. But a joint only works if both faces bond, and the tank side is the one that will not.

Does the sealant need drinking water approval for a rainwater tank?

If the joint contacts stored water that will be drunk, yes. Roof & Gutter Silicone and Roof & Gutter Speed Seal meet AS/NZS 4020, though not all Roof & Gutter colours have been tested and the product is not recommended for continuous submersion. See is Glass Silicone safe for drinking water.

Could I seal it from the inside instead?

The polyethylene surface is the same on both faces, so the adhesion problem is unchanged. The fitting is still the answer.

Is this the same issue as irrigation poly pipe?

Yes, exactly — see sealing leaks on irrigation poly pipe.