Sealant To Stop Fuel Leaks

Is there a sealant that will stop a fuel leak on a lawn edger?

It depends on where the leak is, and the two answers are quite different.

If the leak is at the hose, where it fits over the outlet nipple: replace the hose. Fuel hose becomes brittle and loose over time in contact with fuel, and a hose that has hardened will not seal on the nipple however much you clamp it. A new length of the correct fuel hose is the fix.

If the leak is where the fitting seats into the tank: there is nothing in the Selleys range that will adhere to the plastic and seal the fitting. These fittings and the tanks themselves are often made of plastics that cannot readily be glued, such as polyethylene or polypropylene.

Why the tank fitting cannot be sealed with an adhesive

Small engine fuel tanks are almost always polyethylene or polypropylene, and those two plastics defeat nearly every adhesive technology there is. The exclusion appears on technical data sheet after technical data sheet across the Selleys range:

• Roof & Gutter Silicone — not for use on polyethylene, polypropylene or PTFE.
• Brick & Concrete Silicone — not for use on polyethylene, polypropylene or Teflon.
• Auto Fix Instant Gasket (High Temp) — not suitable for PE, PP or PTFE.
• Knead It Aqua — will not adhere to polyethylene, polypropylene or Teflon.

The reason is the surface itself. Polyethylene and polypropylene have very low surface energy, so adhesives bead up on them instead of wetting out, and they are chemically inert, so there is nothing for an adhesive to react with. It is the same property that makes them good fuel tanks in the first place.

The second problem: fuel resistance

Even setting the plastic aside, a fuel joint is a hard chemical environment, and the sealant data sheets are explicit about it.

Selleys Silicone 401 has good resistance to most organic and inorganic chemicals, solvents and oils — and its limitations then say plainly: not recommended for use with petrol, distillate and some other hydrocarbon solvents. That is a product with a −60°C to +205°C service range and real chemical resistance, and petrol is still outside what it is recommended for.

So a repair to a fuel joint faces two independent problems at once: a substrate nothing bonds to, and a fluid that attacks the sealant. Either one on its own would be enough reason not to attempt it.

The safety argument

This one is worth stating directly. A leaking fuel joint on a petrol engine is a fire risk. A repair that appears to work — a sealant that skins over, looks solid, and holds while the machine is cold and stationary — is arguably worse than no repair, because it removes the visible warning while the underlying leak continues.

Petrol seeps, evaporates and finds ignition sources. Vibration from a two-stroke engine works at any joint continuously. A bond that was marginal to begin with and is being softened by fuel will not last, and its failure will not announce itself.

The correct fix for a fuel system component is a replacement part designed for fuel service: a new hose, a new grommet or bung, a new tank. Small engine spares are widely available and inexpensive.

What about Selleys All Plastics?

Selleys All Plastics is the one product in the range that bonds polyethylene and polypropylene at all. It is a cyanoacrylate supplied as a 3g tube with a surface-activating primer pen, and with the primer applied to both surfaces the published bond strengths are 5.6 MPa on polyethylene and 2.9 MPa on polypropylene.

It is still not the answer here. It is described for repairing toys, games, office items, remote controls and ornaments. It cures rigid, the surfaces have to be clean, dry and close fitting, and nothing in its documentation covers petrol resistance. A fuel tank fitting is not what it is for.

Replacing the fuel hose

Where the leak is at the hose, the repair is straightforward:

1. Let the engine cool completely and work outdoors, away from any ignition source.
2. Drain the fuel into an approved container.
3. Remove the old hose and take it with you as a pattern — hose is sized by internal diameter and wall thickness.
4. Fit new hose rated for fuel service, of the correct size, and secure it as the manufacturer specifies.
5. Refuel, then run the machine and check the joint before putting it away.

Old fuel hose hardens because the plasticisers leach out into the fuel. That is why it goes brittle and loose, and why it is a consumable rather than a permanent part.

Common questions about sealing fuel leaks

Is there any Selleys sealant rated for petrol?

No. Silicone 401 is not recommended for use with petrol, distillate and some other hydrocarbon solvents, and no other product in the range carries a petrol rating.

What if I use a fuel-resistant sealant from elsewhere?

The substrate problem remains. If the tank and fitting are polyethylene or polypropylene, adhesion is the limiting factor regardless of the sealant's fuel resistance.

Would an epoxy putty work?

Knead It Aqua will not adhere to polyethylene, polypropylene or Teflon. It bonds well to glass, ceramics, masonry, metal, timber and fibreglass — but not to the plastics that fuel tanks are made from.

How can I tell what the tank is made of?

Most moulded plastic tanks carry a recycling code or material stamp on the underside or inside the filler neck. PE and PP are the common ones.

My tank is metal, not plastic — does that change things?

The adhesion problem goes away, but the fuel resistance problem does not. A leaking metal tank or fitting is still a job for a replacement part or a specialist repairer, not a general purpose sealant.

What if the leak is only when the tank is full?

That usually points to the seal at the fitting or the filler rather than a crack, and it is still the fitting that needs replacing.

Can I use thread tape on the fitting?

Follow the manufacturer's specification for that fitting. Fuel systems have their own requirements and a general plumbing product is not automatically appropriate.

Is this the same problem as poly irrigation pipe?

The adhesion half of it is identical — see sealing leaks on irrigation poly pipe and sealing a poly rainwater tank and plastic stormwater pipe.