What is Selleys Silicone 401 RTV, and where should you use it?

Selleys Silicone 401 RTV is a one component, ready to use, acid cure silicone adhesive sealant developed for form-in-place gaskets, adhesive sealing and electrical insulating applications in manufacturing and plant maintenance. It is an industrial product rather than a household one, and the specification that defines it is temperature: it remains permanently flexible from -60°C to 205°C, tolerating short periods at 260°C.

The thing to know before choosing it is that the high temperature capability comes with a narrow set of exclusions you must respect. It is not recommended for use with petrol, distillate and some other hydrocarbon solvents, it cannot be over-painted, and it will not cure in a totally confined joint. Chemical compatibility should be tested for each application you are considering — the manufacturer says so explicitly, and on a gasket that is not optional advice.

Technical specification

Property	Typical result
Technology	Acetic cure silicone
Colour	Clear
Skin time	5 minutes
Cure rate	3mm per 24 hours
Shore A hardness	23
Elongation at break	400%
Modulus at 100% elongation	0.365 MPa
Elastic recovery	100%
Service temperature	-60°C to +205°C (260°C intermittent)
Packaging	310g cartridge, clear

These are indicative data only, not intended for the preparation of specifications.

What the key numbers mean in practice

The skin time of 5 minutes is the number that governs how you work. For gasketing, it means the two surfaces must be mated before the skin forms — five minutes is the entire window between laying the bead and closing the joint, so dry-fit the assembly and have your fasteners ready before you open the cartridge. For joint sealing, it means tooling happens immediately and masking tape comes off straight away, not at the end of the run.

The cure rate of 3mm per 24 hours is a separate matter: it describes how deep the cure travels into the bead. A thin gasket film is through-cured quickly, while a thick bead takes days. Because this is a moisture-triggered acetic cure, the sealant needs air contact to cure at all — which is why a minimum adhesive thickness of 0.2mm is specified for gasketing, along with the instruction to ensure some residual external air contact, and why totally confined joints are excluded outright.

Elastic recovery of 100% with 400% elongation at break and a low modulus of 0.365 MPa describes a soft gasket material that returns fully to its original shape after being compressed or stretched. That is exactly what a form-in-place gasket needs across a machine's heating and cooling cycles: a material that takes a set would leave a gap the next time the assembly cooled.

The -60°C to 205°C service range, with short excursions to 260°C, is what separates this product from a general-purpose sealant. Note the distinction between continuous and intermittent — 260°C is described as short periods, not a working temperature.

Where to use it

Silicone 401 RTV suits most metal, glass, ceramic, some rigid plastic, painted and wood substrates, and it offers primerless adhesion to them. Its intended applications are:

• Form-in-place gaskets
• Adhesive sealing
• Electrical insulating applications in manufacturing and plant maintenance

It has good resistance to most organic and inorganic chemicals, solvents and oils — with the hydrocarbon exclusions noted below.

How to apply it

General

1. Work in a well ventilated area and avoid contact with skin and eyes.
2. Make sure the surface is clean and dry.
3. Cut the cartridge, fit and cut the nozzle, and apply with a cartridge gun.

For adhesive or gasketing use

1. Apply the adhesive or sealant to one surface.
2. Mate the surfaces before the skin forms — you have about five minutes.
3. Keep the adhesive thickness to a minimum of 0.2mm.
4. Ensure some residual external air contact so the sealant can cure.

For joint sealing use

1. Mask the adjacent surfaces.
2. Apply the sealant and tool immediately, forcing it into the joint.
3. Remove the masking before the sealant skins. For neater joints, mask both sides before applying and pull the tape off before skinning — tape removed later drags the edge of the bead.

Limitations — where Silicone 401 RTV should not be used

These are the manufacturer's stated limits:

• Not recommended for use with petrol, distillate and some other hydrocarbon solvents. This matters on fuel systems in particular, where the product's general chemical resistance does not extend.
• Do not use in totally confined joints where the sealant is not exposed to atmospheric moisture. Without air contact it will not cure.
• Chemical compatibility should be tested for each application under consideration. Broad chemical resistance is not the same as verified compatibility with the specific fluid your plant handles.
• Cannot be over-painted. Complete any painting seven days before applying the sealant.

The confined-joint limitation is the one that produces the most confusing failures: an assembly that looks properly sealed can be holding uncured silicone inside it indefinitely, because the cure chemistry never got the moisture it needed.

Clean up and storage

Uncured sealant is removed with mineral turps. Cured sealant can only be removed by scraping or trimming with a blade. Given a five-minute skin time, cleaning up as you go is the only workable approach.

Store unopened sealant below 27°C in a dry area, and use within 12 months of purchase.

Hazard classification

Classified as hazardous under Safe Work Australia GHS 7, with the signal word Danger.

• Eye damage — Category 1. H318: causes serious eye damage.
• Skin corrosion/irritation — Category 2. H315: causes skin irritation.

What is in it

Chemical entity	CAS number	Proportion (w/w)
Acetoxysilane	4253-34-3	1–10%
Acetic acid	64-19-7	<1%
Octamethylcyclotetrasiloxane (D4)	556-67-2	<1%
Non-hazardous or below reporting limits	—	Balance

Note the severity: H318, serious eye damage, signal word Danger. That is a step above the Category 2A eye irritation carried by Glass Silicone and No Mould. Uncured acetoxysilane in the eye can cause permanent injury, so a splash needs a 15-minute flush and a call to the Poisons Information Centre — not a rinse and a blink.

This is an acetic cure silicone and the safety data sheet declares free acetic acid in the formulation as well as the acetoxysilane that generates more of it during cure. That is the vinegar smell, and it is why this sealant must not be used on copper-bearing metals, zinc, concrete or cement products, mirror backing or natural stone — acetic acid corrodes all of them.

Ventilate while it cures. In an enclosed space the acetic acid vapour irritates eyes and airways well before the bead has skinned.

Protection

• Gloves — nitrile rubber; essential wherever a sensitisation classification applies.
• Eye protection — chemical goggles or safety glasses.
• 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 immediately and continuously with water for at least 15 minutes, holding the eyelids apart. Remove contact lenses if present and easy to do, then keep rinsing. Immediately call a Poisons Information Centre or doctor. This product causes serious eye damage and the injury is not always obvious at first.
Skin	Wash with plenty of water. Remove contaminated clothing and wash before reuse. If irritation develops, get medical advice.
Inhalation	Move to fresh air. If irritation persists, seek medical advice.
Swallowed	Do not induce vomiting. Rinse the mouth and seek medical advice immediately.

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 Silicone 401 RTV

What temperature can it handle?

It remains permanently flexible from -60°C to 205°C and will tolerate short periods at 260°C. The 260°C figure is intermittent, not a continuous working temperature. There is more background on this in how temperature resistance affects silicone sealant performance.

How long do I have before it skins?

Five minutes. For gasketing you must mate the surfaces inside that window; for joint sealing, tool immediately and remove masking before skinning.

How long does it take to cure?

3mm per 24 hours, working inward from the surface, so it depends on the thickness of the bead.

Can I use it as a gasket between two flat flanges?

Yes, that is its intended use — but keep the adhesive thickness to at least 0.2mm and make sure some part of the sealant stays in contact with the air so it can cure. A totally confined joint will not cure.

Can I use it on a fuel system?

It is not recommended for use with petrol, distillate and some other hydrocarbon solvents. Test chemical compatibility for your specific application before you rely on it.

Can I paint over it?

No. It cannot be over-painted. Complete any painting seven days before applying the sealant.

Is it resistant to oils and solvents?

It has good resistance to most organic and inorganic chemicals, solvents and oils, but the hydrocarbon exclusions above apply, and compatibility should be tested per application.

Does the gasket take a permanent set over time?

Elastic recovery is quoted at 100%, so the cured material is specified to return to its original dimensions.

How do I remove cured product?

By scraping or trimming with a blade. Uncured product comes off with mineral turps.

How should I store it, and how long does it last?

Store unopened below 27°C in a dry area and use within 12 months of purchase.

What size and colour is it?

A 310g cartridge, clear.

Is this the right product for a household bathroom or kitchen joint?

It is developed for manufacturing and plant maintenance. For domestic sealing, look at the wet area silicone range or the broader multi-purpose sealant range.