What is Selleys Auto Fix Marine Sealant, and where should you use it?

Selleys Auto Fix Marine Sealant is a translucent, highly waterproof, neutral curing silicone with good adhesion to marine materials. It cures to a tough, flexible, waterproof seal that is water, weather and salt spray resistant, and it resists temperatures of approximately −60°C to +180°C continuously, and up to +240°C intermittently.

The most important thing to know before you buy it is where it must not go: it is not suitable for below water-line seals unless it is used as a gasket that is held mechanically in compression. It is a sealant for the parts of a boat above the waterline, and for gasketed fittings where a bolted joint carries the load — not a substitute for a proper hull seal.

What the cure times mean in practice

There are three different cure figures, and using the wrong one is the most likely way this job goes wrong.

In normal applications, the sealant cures in 24 hours. That is a bead run into a joint or crack, with air on both sides.

Used as a gasket or an adhesive, up to 7 days may be required for full curing. That is a very large difference, and the reason is simple: a neutral curing silicone cures from the outside in as it takes up moisture from the air. Squeezed between two mating faces, most of the bead has no air to cure from, so it takes days rather than hours.

Used to replace compression gaskets in port-hole closures, leave it to cure a few days before fully tightening the bolts or screws on the compression ring. Tighten too early and you squeeze uncured silicone out of the joint, leaving a gasket thinner than the one you meant to make.

The other timing figure to plan around is the 3-minute mating window for adhesive or gasket use. Apply to one surface and bring the parts together within three minutes — set the job up completely before you open the tube.

Where to use it

It can be used for:

• Joints and cracks
• Bedding of fittings
• Windows and hatches
• Locking nuts, bolts and plates to prevent vibration
• Sealing and waterproofing electrical connections
• Binding and repairing most marine materials

It is suitable for use on most metals, with an important exception noted below. Its salt spray resistance is what separates it from a general-purpose sealant — the marine environment attacks the joint as much as the water does. The auto sealant range covers related jobs on vehicles and caravans.

How to apply it

1. Clean the surfaces with mineral turps and dry them. Salt residue and grease both prevent adhesion, and a marine surface usually has both.
2. Pierce the seal in the tube and fit the nozzle.
3. Extrude the sealant with steady pressure into the joint or onto the surface, pushing it into the joint rather than laying it across the top.
4. For adhesive or gasket use, apply to one surface and mate the parts within 3 minutes.
5. Tool to a smooth finish with a spatula dipped in detergent water. The detergent film stops the silicone dragging on the tool.
6. Clean up before the sealant cures, with mineral turps.
7. Replace the cap after use to prevent the sealant curing in the nozzle.

Limitations — where it should NOT be used

These are the manufacturer's stated limits, and on a boat they matter more than usual:

• Will not adhere to copper, brass or zinc containing metals. This rules out a great many marine fittings — check what the fitting actually is before you bed it.
• Not suitable for polycarbonate, polyethylene or polypropylene plastics. Polycarbonate is a common hatch and window glazing material, so confirm what your hatch is made from.
• The sealant cannot be painted over.
• Not suitable for below water-line seals, unless used as a gasket which is held mechanically in compression.
• Not recommended for trafficable joints, such as open joints between deck planks.
• Not suitable for contact with petrol or distillate.

Two of these are worth restating because they are easy to work around only if you know them in advance. Because the sealant cannot be painted, any painting has to be finished before you seal, not after. And because it is not recommended for trafficable joints, deck seams that get walked on are outside its scope — foot traffic works a bead loose in a way that weather does not.

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.
• Sensitisation — skin — Category 1. H317: may cause an allergic skin reaction.

It is not classified as Dangerous Goods by the criteria of the Australian Code for the Transport of Dangerous Goods by Road & Rail or New Zealand NZS5433.

What is in it

Chemical entity	CAS number	Proportion (w/w)
2-Butanone, O,O',O″-(methylsilylidyne)trioxime	22984-54-9	1–10%
2-Butanone, O,O',O″-(ethenylsilylidyne)trioxime	2224-33-1	<1%
Cyclotetrasiloxane, octamethyl-	556-67-2	<1%
1,2-Ethanediamine, N-[3-(trimethoxysilyl)propyl]-	1760-24-3	<1%
Methyl ethyl ketoxime	96-29-7	<1%
Non-hazardous ingredients	—	Balance

This is the chemistry behind the word "neutral" in the description at the top of the page. The oxime silanes are the crosslinkers, and free methyl ethyl ketoxime is what is left over from that reaction. Oximes are the component class most often associated with skin-sensitisation classifications in neutral cure silicones, which is the obvious chemical reading of H317 here — the safety data sheet does not name a responsible ingredient, so take that as inference, not a sourced attribution.

Sensitisation deserves a sentence of its own, because it is not the same as irritation. An irritant is unpleasant while it is on you and finished when you wash it off. A sensitiser can teach the immune system to react — generally permanently, once acquired — and afterwards far smaller exposures will set it off. It comes from repeated low-level contact rather than one memorable splash, which on a boat means the habit of running a bead and tooling it with a wet finger, deck fitting after deck fitting.

The setting is worth thinking about too. Marine sealing is frequently done in a cabin, a bilge, a locker or under a foredeck — small, enclosed, awkward spaces where your face is close to the work and your eyes are directly above it. H319 is a Category 2A eye irritation, which is reversible, but the safety data sheet's own advice for this product is to rinse cautiously for several minutes, remove contact lenses if you can, and keep rinsing. That is hard to do properly at a mooring. Safety glasses are the cheap version of that problem.

Protection

• Eye protection — safety glasses, particularly for overhead and confined work below decks.
• Gloves — nitrile rubber is suitable for intermittent contact.
• Respiratory — the safety data sheet carries P261, avoid breathing dust, fume, gas, mist, vapours or spray, and recommends a suitable respirator where that cannot be achieved by ventilation. In a cabin or locker, ventilate first.
• Overalls and safety shoes; avoid contact with clothing.
• 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. Remove contact lenses if present and easy to do, then continue rinsing.
Skin	Effects may be delayed. Remove contaminated clothing immediately and flush skin and hair with running water. If swelling, redness, blistering or irritation develops, seek medical assistance.
Inhalation	Remove the person from exposure without becoming a casualty yourself. Loosen tight clothing, keep them comfortable, warm and at rest until recovered. Seek medical advice if effects persist.
Swallowed	Rinse the mouth with water. Do NOT induce vomiting. Give a glass of water. Never give anything by mouth to an unconscious person. 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 Auto Fix Marine Sealant

How long does it take to cure?

24 hours in normal applications. Used as a gasket or adhesive, up to 7 days may be required for full curing.

Can I use it below the waterline?

Not for below water-line seals, unless it is used as a gasket held mechanically in compression.

Can I paint over it?

No. The sealant cannot be painted over, so complete any painting first.

Will it stick to brass or bronze fittings?

It will not adhere to copper, brass or zinc containing metals.

Can I use it on a polycarbonate hatch?

No. It is not suitable for polycarbonate, polyethylene or polypropylene plastics.

How hot can it get?

It resists approximately −60°C to +180°C continuously, and up to +240°C intermittently.

Will it stand up to salt water spray?

Yes — it is water, weather and salt spray resistant.

Can I use it on deck seams?

It is not recommended for trafficable joints, such as open joints between deck planks.

Can I use it near fuel?

No. It is not suitable for contact with petrol or distillate.

How long do I have to join two parts?

Three minutes, when using it as an adhesive or gasket. Apply to one surface and mate within that time.

I am replacing a port-hole gasket — when can I tighten the bolts?

Leave it to cure for a few days before fully tightening the bolts or screws on the compression ring.

Can I seal electrical connections with it?

Yes — sealing and waterproofing electrical connections is one of its listed uses.

How do I get a neat finish?

Tool it with a spatula dipped in detergent water before it cures.

How do I clean up?

With mineral turps, before the sealant cures.