What is Selleys Marine Flex, and where should you use it?
Selleys Marine Flex is a high performance adhesive sealant built on Sil-X polymer, made for boats, pools and spas where you need a waterproof seal or bond. Its defining property is that it can be applied and will cure under water — you do not have to drain a pool or spa to make a repair. Compared to traditional polyurethane, Selleys states it provides a more durable seal and bond with superior UV weather resistance.
Before you choose it, read the limitations at the bottom of this page carefully. Curing underwater is not the same as being rated for permanent immersion, and the manufacturer draws that line explicitly. Marine Flex is also solvent-free and isocyanate-free, shock absorbing and vibration dampening, sandable and paintable, and non-sagging.
Technical specification
| Property | Typical result |
|---|---|
| Technology | Sil-X |
| Skin formation | Approximately 40 minutes |
| Curing rate | Approximately 2mm per 24 hours |
| Service temperature (in water) | 0°C to 40°C continual exposure 40°C to 70°C short term exposure |
| Shrinkage | 0% |
| Slump | <1mm |
| Hardness | 75 (Shore A) |
| Modulus | 1.45 MPa |
| Elongation at break | Up to 30% |
| Shelf life | 12 months |
Supplied in 410 gram cartridges, in white and black. Indicative data only, not intended for the preparation of specifications.
Selleys attaches an important qualifier to the skin and cure figures: expect longer skin formation and slower curing at cold temperatures, and shorter skin formation and faster curing in wet conditions or submersion.
What the key numbers mean in practice
Skin formation approximately 40 minutes, but tool within 15 minutes. The directions ask you to smooth off with a spatula within 15 minutes of applying, well inside the 40-minute skin figure. Treat 15 minutes as your working window and 40 minutes as the point of no return.
Curing rate approximately 2mm per 24 hours. The sealant cures inward from the exposed surface at roughly 2mm a day, which means a deep bead takes several days to cure through even though it feels firm within hours. Selleys advises avoiding subjecting the sealant to water pressure or movement until fully cured — on a hull seam or a pool joint of any depth, that is a multi-day wait, not an overnight one. The one thing working in your favour is that submersion and wet conditions speed the cure rather than slowing it.
0% shrinkage and slump under 1mm. The bead you tool is the bead you keep — it will not pull back from the joint faces as it cures, and it will not sag on a vertical or overhead surface. That matters on a topside seam where a shrinking sealant would open a hairline gap along the edge.
Service temperature in water: 0°C to 40°C continual, 40°C to 70°C short term. A heated spa sitting continuously in the high 30s is inside the continual range; sustained operation above 40°C is not.
Where to use it
Marine Flex can be used on the following surfaces, wet or dry, 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 or primed and painted. Porous substrates need that barrier before the sealant goes on.
Selleys states Marine Flex can be applied to fully dried alkyd enamel, oil-based, water-based and epoxy paints, including epoxy pool and boat paints — while recommending you confirm paintability and adhesion with a small sample first.
The technical features Selleys claims are: watertight and waterproof, suitable for above and below the waterline, no primer required on the listed surfaces, shock absorbing and vibration dampening, solvent-free and isocyanate-free, sandable and paintable, weather and UV resistant, indoor and outdoor, and non-sagging.
How to apply it
- Ensure surfaces are clean and free from oil, grease and loose particles prior to application.
- Lightly sand plastic surfaces before application to enhance adhesive strength. Gelcoat must be lightly sanded prior to application.
- Wipe metal and plastic surfaces over with a solvent such as mineral turps to remove all traces of oil and mould release residue left from the fabrication process. Mould release is the usual reason a sealant fails on new fibreglass or moulded plastic — it is invisible and it stops adhesion.
- Apply with a caulking gun, pushing the sealant into the joint or gap rather than laying it across the surface.
- Smooth off using a spatula within 15 minutes of applying.
- Leave it alone. Avoid subjecting the sealant to water pressure or movement until fully cured.
For neater joints, mask both sides before applying and remove the tape before the sealant skins. If you are painting over it and the paint does not wet out the sealant surface properly, lightly sand the sealant before painting.
Limitations — where Marine Flex should not be used
These are the manufacturer's stated limits and they are the most important part of this page:
- Not suitable for PE, PP or PTFE.
- Not suitable for permanent water immersion.
- Not suitable for plugging leaks subject to pressure.
- Not for polycarbonate and Perspex under water. Polycarbonate is on the no-primer surface list, but that does not extend to underwater use.
- It cannot be used on chlorinated rubber-based pool paint.
- It cannot be used to repair flexible vinyl pool liners or inflatable pools.
Selleys also advises that critical seals or repairs underwater should be regularly checked and repaired if required. Treat an underwater repair as something to inspect, not something to forget about.
The 10 year guarantee, and its conditions
Selleys guarantees Marine Flex to provide a watertight seal for at least 10 years — but the conditions are specific. It applies when the product is used as directed, in above the water-line applications only, within 12 months of purchase, in joints configured with tight fitting non-absorbent backing material as per the directions for optimal joint design, with joint width double the joint depth, joint depth at least 8mm, and joint width 40mm or less.
Those dimensions are worth designing to if the guarantee matters to you. A properly formed joint with correct backing qualifies; a thin smear across a surface does not. If the product does not perform as specified within the warranty period, Selleys will replace it free of charge on proof of purchase. These benefits are in addition to rights and remedies available under Australian Consumer Law.
Clean up and storage
Clean up uncured Marine Flex with rags wetted with mineral turps. Cured sealant can be removed by scraping or sanding. Store in cool, dry conditions below 30°C, and use within 6 months of opening. Shelf life is 12 months.
Hazard classification
Classified as hazardous under Safe Work Australia GHS 7, with the signal word Warning.
- Eye damage/irritation — Category 2A. H319: causes serious eye irritation.
Not classified as Dangerous Goods for transport.
What is in it
| Chemical entity | CAS number | Proportion (w/w) |
|---|---|---|
| 3-(Trimethoxysilyl)-1-propanamine | 13822-56-5 | 1–10% |
| Bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate | 41556-26-7 | <1% |
| Methyl (1,2,2,6,6-pentamethyl-4-piperidyl) sebacate | 82919-37-7 | <1% |
| Non-hazardous or below reporting limits | — | Balance |
Marine Flex is the mildest hazard profile of any product in this part of the range: a single classification, eye irritation, and it is not Dangerous Goods. There is no solvent load, no flammability classification and no skin sensitisation classification — a consequence of the Sil-X hybrid polymer chemistry, which cures with atmospheric moisture without releasing acetic acid.
The two piperidyl sebacates at under one percent each are hindered amine light stabilisers. They are what keep the cured sealant from degrading under UV, which is the point of a product intended for marine and exterior use.
Eye protection still matters, because this product is frequently applied overhead and into vertical joints on a hull or a cabin roof, where a cartridge under pressure can spit.
Protection
- Gloves — nitrile rubber for intermittent contact.
- Eye protection — safety glasses or goggles.
- Ventilation — work in a well-ventilated area; do not work in a confined space.
- Wash hands before eating, drinking or smoking, and wash contaminated clothing before reuse.
First aid
| Exposure | What to do |
|---|---|
| Eyes | Rinse cautiously with water for several minutes, removing contact lenses if present and easy to do, then keep rinsing. If irritation persists, get medical advice. |
| Skin | Wash with soap and water. If a rash, redness or itching develops — including some hours later — get medical advice, as this may indicate an allergic reaction rather than simple irritation. |
| Inhalation | Move to fresh air. If irritation persists, seek medical advice. |
| Swallowed | Do not induce vomiting. Rinse the mouth and seek medical advice. |
If poisoning occurs, contact a doctor or the Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand — with the container to hand. Selleys emergency line: 1800 220 770 in Australia, 0800 220 770 in New Zealand.
Common questions about Selleys Marine Flex
Can I really apply it underwater?
Yes. Selleys states it can be applied and will cure under water, with no need to drain your pool or spa. Cure is actually faster in wet conditions or submersion.
If it cures underwater, why is permanent immersion listed as a limitation?
Because those are two different things. The manufacturer states the product can be applied and cured under water, and separately lists permanent water immersion among its limitations, along with plugging leaks subject to pressure. Both statements come from the same technical data sheet, so read the limitation as the binding one and treat underwater application as a repair method rather than a permanent-immersion rating.
How long before the repair can take water pressure?
The cure rate is approximately 2mm per 24 hours, and Selleys says to avoid subjecting the sealant to water pressure or movement until fully cured. A deeper bead needs proportionally longer.
Do I need a primer?
Not on aluminium, galvanised steel, ceramic tiles, fibreglass/FRP, gelcoat, polycarbonate, glass or most plastics. Timber and concrete must be sealed or primed and painted first.
Do I need to sand first?
Gelcoat must be lightly sanded before application, and plastic surfaces should be lightly sanded to enhance adhesive strength.
Can I paint over it?
Yes — it is sandable and paintable. It can be applied to fully dried alkyd enamel, oil-based, water-based and epoxy paints, including epoxy pool and boat paints. Confirm with a small sample first, and if paint does not wet out the surface properly, lightly sand the sealant before painting.
Can I use it on a vinyl pool liner?
No. It cannot be used to repair flexible vinyl pool liners or inflatable pools.
Can I use it on painted pool surfaces?
Not on chlorinated rubber-based pool paint.
Will it stick to plastic?
Most plastics, but pre-test before use. It is not suitable for PE, PP or PTFE, and not for polycarbonate or Perspex under water.
Will it sag on a vertical joint?
Slump is under 1mm and shrinkage is 0%, so the bead holds its shape and does not pull back as it cures.
How hot can it get?
In water, 0°C to 40°C continual exposure and 40°C to 70°C short term exposure.
What sizes and colours are available?
410 gram cartridges in white and black.
How do I clean it up?
Uncured product comes off with rags wetted with mineral turps. Cured sealant must be scraped or sanded off.
For related jobs, see how to seal a leak in a boat and how to repair loose pool tiles, or browse the specialist glue range.
What is Selleys Marine Flex used for?
Selleys Marine Flex is a specialist waterproof adhesive sealant designed to protect boats from water damage by sealing and bonding surfaces both above and below the waterline.
What technology does Selleys Marine Flex use?
Selleys Marine Flex uses Sil-X advanced polymer technology.
What sizes and colours is Selleys Marine Flex available in?
Selleys Marine Flex is available in 410g cartridges in White and Black colours.
Can Selleys Marine Flex be applied underwater?
Yes, Selleys Marine Flex can be applied underwater or to damp surfaces, and it cures underwater, with full cure time depending on the thickness of the bead applied.
What surfaces is Selleys Marine Flex suitable for?
Selleys Marine Flex is suitable for metal, glass, sealed timber, sealed concrete, tiles, fibreglass, gelcoat, and most plastics (though plastics should be tested first).
What surfaces or materials are not suitable for Selleys Marine Flex?
Selleys Marine Flex is not suitable for polyethylene, polypropylene, or Teflon, and gelcoat must be sanded before use.
How should porous or certain substrates be prepared before using Selleys Marine Flex?
Porous substrates such as timber and concrete should be sealed or painted before use, and fibreglass, gelcoat, or polycarbonate surfaces should be lightly sanded first.
What are the directions for using Selleys Marine Flex?
According to the directions for use: 1. Ensure surfaces are clean and free from oil or grease. 2. Cut the cartridge and nozzle, then apply a bead. 3. Smooth or tool the bead before it skins over. 4. The product cures underwater, and full cure depends on the thickness of the bead applied.