N Mastic

What is Selleys N-Mastic, and where should you use it?

Selleys N-Mastic is a non-skinning, polybutene-based compound that stays permanently soft and paste-like. It is off-white, supplied in a 300mL cartridge, and it is designed for one particular kind of job: sealing concealed lap joints between building materials to keep draughts and water out.

The defining characteristic is the one that decides whether it suits your job. N-Mastic never cures. It does not skin over, it does not go hard, and it does not turn into a rubber. That is exactly why it works in a concealed lap joint — it stays soft, stays in contact with both faces and keeps sealing as the materials shift — and exactly why Selleys says it must not be used in an exposed joint, where a permanently tacky surface picks up dirt and gets touched.

Technical specification

Property	Typical result
Technology	Polybutene
Colour	Off-White
Density	1.4
Service temperature	−40°C to +80°C
VOC content	135 g/L
Packaging	300mL cartridge
Shelf life	24 months from date of manufacture, unopened

These are indicative data from the product's technical data sheet only, and are not intended for the preparation of specifications.

What the key numbers mean in practice

The service temperature range of −40°C to +80°C is unusually wide, and it is the reason N-Mastic turns up in cool room installation. A sealant that has to work at a cool room panel joint spends its life at one end of that range; the same product on a sun-exposed prefabricated panel can sit near the other. Because the compound does not cure, it does not have a cure schedule that cold weather interferes with — it is soft when you gun it and soft afterwards.

Because it does not cure, there is no waiting. There is no skin time to work within, no tooling window that closes, and no cure rate that dictates when the next trade can start. The joint is doing its job as soon as it is filled and the components are fixed together. The trade-off is that the seal is never mechanically tough: nothing hardens, so nothing resists being disturbed. That is a fair exchange in a concealed joint and a poor one anywhere a hand or a broom can reach.

Low shrinkage matters more in a lap joint than it sounds. A compound that shrank as it dried would pull away from one face of a lap joint and open a path for moisture. N-Mastic has nothing to lose — no solvent flashing off, no water evaporating — so the bead you place is the bead you keep.

Where to use it

Selleys states N-Mastic is suitable for sealing concealed lap joints between:

• Plastics
• Metals
• Timber
• Glass
• Masonry
• Expanded foam panels

The specific installations named in the technical data sheet are cool rooms, prefabricated construction and gasket sealed windscreens, in each case to exclude draughts and water penetration through joints in materials.

The common thread is a joint where two sheets or panels overlap, the sealant is trapped between them, and the assembly is then mechanically fixed. It is a bedding compound rather than a gap filler. If what you actually have is a visible gap that needs to be filled, tooled and painted, that is a different product category — look at the gap filler range or the broader sealants range instead.

How to apply it

Selleys is explicit that its directions are a guide only, and that the installer should apply all Australian Standards, BCA/NZBC requirements and best practice when installing.

1. Prepare the surfaces. Surfaces to be sealed must be clean, dry and free from dust, grease and flaking paint.
2. Clean the surfaces with mineral turpentine and allow them to dry. Because the compound never cures, this is your only opportunity to get a clean bond — there is no chemical grab later to make up for a dusty face.
3. Apply N-Mastic using a cartridge gun, and tool it if necessary.
4. Mechanically fix the joints where necessary. This step is part of the system, not an optional extra. A non-hardening compound contributes no structural hold of its own; the fixings hold the joint together and the mastic seals it.

Limitations — where N-Mastic should not be used

These are the manufacturer's stated limits, and with a non-curing compound they define the product more sharply than the feature list does:

• Not for exposed joints. Because the sealant is non-skinning and non-hardening, it should not be used where dirt pick-up or hand contact can be a problem. A permanently soft bead in a trafficked location will collect grime and transfer to whatever touches it.
• Not for areas subject to considerable dynamic stress. N-Mastic is not elastomeric — it does not behave like a cured rubber that stretches and recovers.
• Where static pressure resistance is critical, a curing sealant is recommended instead. N-Mastic is resistant to static pressures, but the technical data sheet still directs you to a curing product where that resistance is the critical factor.
• Not recommended for sealing lap joints in mobile homes, truck bodies and similar, because significant vibration and flexing occur during travel.

That last point is worth spelling out, because a truck body looks superficially like exactly the kind of panelised lap-jointed structure N-Mastic is made for. The difference is that it moves constantly. A cool room panel joint is essentially static; a truck body joint flexes every time the vehicle takes a corner or a bump. The compound is ruled out by the movement, not by the joint geometry.

Clean up and storage

Clean up: spills and tools can be cleaned up with mineral turps — the same solvent used for surface preparation, so one bottle covers both ends of the job.

Storage: store unopened cartridges below 30°C in a cool, dry place away from direct sunlight. Unopened cartridges can be stored for 24 months from the date of manufacture.

Note that the shelf life is measured from manufacture, not from purchase. If a cartridge has been sitting in the back of a van through a couple of summers, its date, not its appearance, is what tells you whether it is still within specification — and heat is the thing the storage instruction is protecting it from.

Why "non-skinning" is the whole product

Most sealants are sold on what happens after application: they skin, they cure, they become a flexible rubber that bridges a moving joint. N-Mastic is sold on the opposite. Its stated features are non-hardening (it remains permanently flexible to provide a lasting seal), good adhesion to most substrates, moisture resistance as an excellent barrier to moisture vapour, resistance to microbial attack, low shrinkage and wide service temperatures.

Read as a set, those describe a compound built to sit sealed inside an assembly for years without changing. Resistance to microbial attack matters because a concealed joint cannot be inspected or cleaned. The moisture vapour barrier matters because the point of a cool room or prefabricated panel joint is to stop vapour moving through the building envelope, not just liquid water. And permanent flexibility matters because the joint will never be re-sealed — whatever you bed it with has to still be in contact with both faces in a decade.

For safety, first aid and shipping information, Selleys directs you to the product's safety data sheet.

Hazard classification

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

• Flammable liquid — Category 3. H226: flammable liquid and vapour.

What is in it

Chemical entity	CAS number	Proportion (w/w)
Naphtha, petroleum, hydrotreated heavy	64742-48-9	1–10%
Non-hazardous or below reporting limits	—	Balance

N-Mastic is an off-white paste with a solvent odour, specific gravity around 1.4, and insoluble in water. The hydrotreated heavy naphtha is what gives it that smell and what earns the flammability classification.

Category 3 flammable is a middle grade — it needs more heat to ignite than a Category 2 solvent adhesive like Liquid Nails, but it is not inert. Keep it away from naked flame, hot surfaces and sparks, do not smoke while using it, and ventilate the space. The hazard is the vapour, not the paste.

There is no eye, skin or sensitisation classification on this product, which makes it unusual among the specialist fillers — most of that group carries H317.

Protection

• Gloves — nitrile rubber for intermittent contact.
• Eye protection — safety glasses or goggles.
• Ventilation — work in a well-ventilated area.
• Wash hands after use, 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 hours later — get medical advice, as that may indicate sensitisation 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 N-Mastic

Does N-Mastic ever go hard?

No. It is a non-skinning compound that remains permanently soft and paste-like. That is its intended behaviour, not a fault.

Can I use it in a visible joint?

No. Selleys states it should not be used in exposed joints where dirt pick-up or hand contact can be a problem.

What size does it come in?

A 300mL cartridge, in off-white.

What surfaces will it seal between?

Plastics, metals, timber, glass, masonry and expanded foam panels — in concealed lap joints.

Can I use it on a caravan, truck body or mobile home?

It is not recommended for sealing lap joints in mobile homes, truck bodies and similar, because of the significant vibration and flexing that occurs during travel.

Will it handle a moving joint?

It is not elastomeric and should not be used in areas subject to considerable dynamic stress.

What temperatures will it tolerate?

The service temperature range is −40°C to +80°C.

How do I prepare the surface?

Surfaces must be clean, dry and free from dust, grease and flaking paint. Clean them with mineral turpentine and let them dry before applying.

How do I apply it?

With a cartridge gun, tooling if necessary, then mechanically fix the joint where required.

How do I clean up?

Mineral turps for both spills and tools.

How long can I keep it?

Unopened cartridges can be stored for 24 months from the date of manufacture, below 30°C in a cool dry place away from direct sunlight.

Do I still need mechanical fixings?

Yes, where necessary — the directions include mechanically fixing joints. N-Mastic seals the joint; it is not there to hold the assembly together.