How To Seal a Leak in a Boat
How do you seal a leak in a boat, and which Selleys product does the job?
Boats develop minor leaks over time — gaps in a seam, loose rivets, perished O-rings — and the fix is usually straightforward: find the leak, clean the area, and seal it with Selleys MarineFlex. MarineFlex is a high-performance adhesive sealant that can be applied to wet surfaces and under water, and it will cure under water. Compared with traditional polyurethanes it gives a more durable seal with superior UV resistance.
The one thing to understand before you start is the difference between sealing and plugging. MarineFlex will seal a seam, a rivet line or a hairline crack. It is not rated to plug a leak that is subject to pressure, and it will not withstand strong pressure until it has fully cured. If water is being forced through the hole rather than seeping through it, this is not the repair you want.
What MarineFlex actually is
MarineFlex uses Sil-X technology. It is solvent-free and isocyanate-free, non-sagging, shock absorbing and vibration dampening, sandable and paintable, and rated for indoor and outdoor use. It comes in 410 gram cartridges in white and black.
| Technology | Sil-X |
| Packaging | 410 g cartridge, white or black |
| Skin formation | Approx. 40 minutes |
| Curing rate | Approx. 2 mm / 24 hours |
| Service temperature in water | 0°C to 40°C continual exposure; 40°C to 70°C short term |
| Shrinkage | 0% |
| Slump | <1 mm |
| Hardness | 75 (Shore A) |
| Modulus | 1.45 MPa |
| Elongation at break | Up to 30% |
| Shelf life | 12 months |
These are indicative figures only and are not intended for preparing specifications.
What the numbers mean for a boat repair
Skin formation at about 40 minutes is your working window. Everything that involves shaping or smoothing the bead has to happen inside it, and the data sheet narrows that further by asking you to smooth off with a spatula within 15 minutes of applying. Expect a shorter skin time in wet conditions or submersion, and a longer one in the cold.
A cure rate of about 2 mm per 24 hours is the figure that decides when the boat can go back in the water under load. It is a depth rate, not a total: a 6 mm deep bead in a seam is three days' work for the chemistry, not one. Curing is faster in wet conditions and submersion, and slower at cold temperatures. Until it is fully cured, avoid subjecting the repair to water pressure or movement.
Elongation at break of up to 30% tells you this is a firm, high-modulus sealant rather than a stretchy one. That suits hull seams and rivets, where you want a tough seal that resists abrasion. It is not a high-movement joint sealant.
What MarineFlex sticks to
MarineFlex can be used on the following surfaces without a primer: aluminium, galvanised steel, ceramic tiles, fibreglass/FRP, gelcoat, polycarbonate, glass, and most plastics (pre-test before use).
It can be used on timber and concrete only when they are sealed, primed and painted. On a boat, that means bare timber needs finishing first. MarineFlex can be applied to fully dried alkyd enamel, oil-based, water-based and epoxy paints, including epoxy pool and boat paints — though it is worth confirming adhesion on a small sample before you commit to a long seam.
Fibreglass, gelcoat, polycarbonate and Perspex should be lightly sanded and wiped with turps before application. More generally, the data sheet asks that plastic surfaces be lightly sanded to improve adhesive strength, and that metal and plastic surfaces be wiped over with a solvent such as mineral turps to remove all traces of oil and mould release residue from the manufacturing process. That mould release is invisible and it is the most common reason a bond on a new fitting fails.
How to seal the leak
- Locate the leak. Get the boat onto land and use a garden hose or similar to fill the boat with water. Looking at the boat from the outside, you should be able to see where it is leaking.
- Clean the surface. Make sure it is free of oil, grease, dirt and dust. The surface does not have to be dry — MarineFlex can be applied to wet surfaces and under water — although it cannot withstand strong pressure until it has fully cured.
- Prepare the cartridge. Cut the cartridge tip, fit the nozzle and cut it to size, then insert it into the caulking gun.
- Apply the product to the crack or hole to seal the leak, pushing the sealant into the gap rather than laying it across the surface.
- Smooth it off with a cloth if you are working under water, or with a soapy solution above water, before skinning occurs.
- Leave it to cure before subjecting the repair to water pressure or movement, and check the product's technical data sheet for anything specific to your situation.
For neater joints, mask both sides of the seam before applying the sealant and remove the tape before the sealant skins.
Limitations — where MarineFlex should not be used
- Not suitable for polyethylene, polypropylene or PTFE. Pre-test any other plastic before use.
- Not for permanent water immersion, and not for plugging leaks subject to pressure. This is the limitation to read most carefully. MarineFlex is rated for above and below waterline sealing and will cure submerged, but the data sheet lists permanent immersion among its limitations, and its 10-year watertight guarantee applies to above-the-waterline applications only.
- Not for polycarbonate and Perspex under water. Both are fine above the waterline once lightly sanded and wiped with turps.
- Cannot be used on chlorinated rubber-based pool paint.
- Cannot be used to repair flexible vinyl pool liners or inflatable pools.
- Critical seals and repairs made under water should be regularly checked and repaired if required. Build that inspection into your normal maintenance rather than treating the repair as permanent.
Painting, clean up and storage
MarineFlex is sandable and paintable. If the paint does not wet out the sealant surface properly, lightly sand the cured sealant before painting.
Clean uncured MarineFlex off tools and hands with rags wetted with mineral turps. Once cured, it has to be removed by scraping or sanding, so it is far quicker to clean up while the product is still wet.
Store the cartridge in cool, dry conditions below 30°C, and use it within 6 months of opening. Unopened shelf life is 12 months.
Common questions about sealing a boat leak
Does the hull have to be dry before I apply MarineFlex?
No. MarineFlex can be applied to wet surfaces and under water. It does need to be clean and free of oil, grease, dirt and dust.
Can I apply it while the boat is in the water?
Yes — it can be applied and will cure under water. Smooth the bead off with a cloth rather than a soapy solution if you are working submerged.
How long until the repair will take water pressure?
Until it is fully cured. The cure rate is approximately 2 mm per 24 hours, so a deeper bead takes proportionally longer. Avoid subjecting the repair to water pressure or movement until then. Note that MarineFlex is not rated for plugging leaks subject to pressure at all.
Will it stick to my aluminium hull?
Yes. Aluminium is one of the surfaces listed as needing no primer, along with galvanised steel, fibreglass, gelcoat, glass, polycarbonate and ceramic tile. Wipe the metal with mineral turps first to remove oil and manufacturing residue.
What about a timber boat?
Timber must be sealed, primed and painted before MarineFlex is applied. It will bond to fully dried alkyd enamel, oil-based, water-based and epoxy paints, including epoxy boat paints, but confirm adhesion on a small sample first.
Can I use it on a fibreglass hull?
Yes. Fibreglass, gelcoat, polycarbonate and Perspex should be lightly sanded and wiped with turps before you apply the product.
Will it work on plastic fittings?
Most plastics are suitable if pre-tested, but polyethylene, polypropylene and PTFE are not. Lightly sand plastic surfaces first.
How long do I have to smooth the bead?
Skin formation is approximately 40 minutes, and the data sheet asks you to smooth off with a spatula within 15 minutes of applying. Work in shorter sections if the run is long.
Can I paint over the repair?
Yes, MarineFlex is sandable and paintable. If the paint will not wet out the sealant, lightly sand the cured surface first.
Is this a permanent fix?
Above the waterline, MarineFlex carries a 10-year watertight guarantee when used as directed within 12 months of purchase and in joints of the design the data sheet specifies. Below the waterline, treat it as a repair you inspect regularly and re-do if required.
How do I get uncured sealant off my hands and tools?
Rags wetted with mineral turps. Cured sealant has to be scraped or sanded off.
How long does an opened cartridge last?
Use it within 6 months of opening. Store it below 30°C in cool, dry conditions. Unopened shelf life is 12 months.
Can I use the same product to re-stick a loose pool tile?
Yes — that is a documented use for MarineFlex, and it does not require draining the pool. See how to repair loose pool tiles.
What product does Selleys recommend for sealing a leak in a boat?
Selleys recommends Selleys MarineFlex for sealing a leak in a boat.
How do you locate a leak in a boat before repairing it?
Get the boat onto land and use a garden hose or other implement to fill the boat with water, then look at the boat from the outside to identify where it is leaking.
Does the boat surface need to be dry before applying MarineFlex?
No, the surface does not have to be dry — MarineFlex can be applied to wet surfaces and even under water, although it cannot withstand strong pressure until it has fully cured.
What surface preparation is needed before applying MarineFlex to a boat leak?
The surface should be clean and free of oil, grease, dirt or dust before applying MarineFlex.
What are the steps for applying MarineFlex to fix a boat leak?
First locate the leak, then clean the surface, cut the cartridge tip and fit a nozzle cut to size, insert it into a caulking gun, apply the product to the crack or hole to seal the leak, and smooth it off with a cloth (if underwater) or a soapy solution before skinning occurs.
How should MarineFlex be smoothed off after application?
According to the page, smooth it off with a cloth if the repair is underwater, or use a soapy solution before skinning occurs.
How long does the whole boat leak repair process take according to the guide?
The guide's total estimated time for the process is 60 minutes.
What common issues cause boats to develop leaks over time?
According to the page, leaks can develop over time from gaps in a seam, loose rivets, or perished o-rings that allow water to filter in.
Is there an alternative product for emergency underwater boat repairs mentioned in the knowledge base?
Yes, Selleys Knead It Aqua is mentioned as an alternative for emergency in-water repairs; it is a hand-kneadable epoxy that can be used on damp, wet, or underwater substrates to patch dings, scratches, cuts, gouges, and holes in wooden boat decks and hulls, and can also repair leaking gaskets.
What preparation is recommended for metal or plastic boat surfaces before sealing, per the knowledge base?
Metal and plastic surfaces should be wiped with a solvent such as mineral turps to remove all traces of oil and mould release residue, and plastic surfaces should be lightly sanded prior to application to enhance adhesive strength.