What is Selleys No Mould Silicone Sealant, and where should you use it?

Selleys No Mould is an acetic cure silicone sealant designed for permanently damp areas where mould would normally take hold — showers, bathrooms, kitchens and laundries. Its distinguishing feature is Speed Seal technology, which gives a waterproof, shower-ready seal one hour after application, against the 24 hours a conventional wet-area silicone needs before it can be exposed to water.

The trade-off to understand before you buy: shower-ready in one hour is not the same as fully cured. The bead reaches a watertight skin quickly, but it must not be touched for at least 48 hours while it cures through.

Technical specification

Property	Typical result
Technology	Acetic cure silicone sealant
Skin time	20 minutes
Cure rate	2mm per 24 hours
Shower ready	1 hour after application
Full cure	At least 48 hours — do not touch before this
Joint movement capability (ISO 9047)	±25%
Elongation (ASTM D412 modified)	Approximately 400%

Available as a 290mL cartridge in transparent and white, and a 100mL tube. Figures above are indicative data from the product's technical data sheet, not intended for the preparation of specifications.

What the cure rate actually means for your joint

A cure rate of 2mm per 24 hours is the number most people misread. It does not mean the bead is finished in a day — it means the silicone cures inward from the surface at roughly 2mm each day. A shallow 3mm bead in a tile corner is through-cured in under two days; a deeper joint takes proportionally longer.

This is why the 48-hour figure is a minimum rather than a guarantee. Where you have filled a genuinely deep gap, give it longer before subjecting the joint to movement or scrubbing.

Joint movement capability of ±25% is the other number worth knowing. It means the cured bead tolerates the joint widening or narrowing by a quarter of its width without failing — enough for the normal thermal and structural movement of a wet area, provided the bead has real depth in the joint rather than being smeared across the surface.

Where to use it

No Mould is built for humid and damp environments where mould is normally present:

• Shower recesses and bath surrounds
• Bathroom and laundry joints between tile, glass and acrylic
• Kitchen splashback and benchtop joints
• Around basins, sinks and toilet bases

Note the important qualifier Selleys itself attaches to the mould-resistance claim: the product stops mould growth provided soap scum and dirt are regularly cleaned off. The mould inhibitor works on the silicone; it cannot stop a colony feeding on the residue that collects on top of it.

Acetic cure — what it means for your surfaces

No Mould is an acetic cure silicone. It releases acetic acid as it cures, which is the sharp vinegar smell present for the first day, and it bonds strongly to glazed ceramic tile, glass and porcelain — the materials a standard shower recess is made from.

That chemistry is also the source of its limitations. Acetic cure is not suitable for metals other than aluminium and stainless steel, because the acid can corrode susceptible metals as it cures. Where your joint involves other metals, natural or engineered stone, or a mirror's silvered backing, a neutral cure silicone such as Selleys Wet Area Silicone is the correct choice instead.

How to apply it

1. Kill any residual mould first. Spray Selleys Rapid Mould Killer into the gap before you seal. Sealing over a live colony traps it against a food source, and it will grow back through the new bead.
2. Clean and dry the surface thoroughly. Wipe the joint with turps and let it dry completely. Silicone will not bond reliably to a damp or greasy surface.
3. Cut the nozzle at 45 degrees, cut the cartridge tip and fit the nozzle. Size the opening to the joint width — you can widen it, but you cannot make it smaller again.
4. Apply with steady pressure from a caulking gun, ensuring the sealant is pushed into the joint rather than laid across the surface.
5. Tool within 10 minutes using a spatula dipped in detergent water. The skin time is 20 minutes; once skinning begins, tooling tears the surface instead of smoothing it.
6. Clean up with turps before the product skins.
7. Wait. Shower use is fine after 1 hour. Do not touch the semi-cured bead until it has fully cured — at least 48 hours.

Limitations — where No Mould should not be used

These are the manufacturer's stated limits, and they matter more than the feature list:

• Not paintable. Complete any painting seven days before applying the sealant.
• Not suitable for aquariums.
• Not suitable for contact with food or drinking water.
• Not suitable for permanent water immersion — it is a wet-area sealant, not an underwater one.
• Not suitable for metals other than aluminium and stainless steel.
• Not suitable for PE, PP or PTFE plastics, which are low-surface-energy materials that silicone will not grip.

If your job falls into any of these, the product to reach for is a different one — not this one applied more carefully.

Clean up and removal

Uncured product comes off with mineral turpentine, which is why cleaning up before the bead skins is worth the minute it takes. Once cured, silicone has to be removed mechanically — cut and scrape it out, or soften it first with a dedicated silicone remover.

For a neater finish, mask both sides of the joint before applying and pull the tape off before the sealant skins. Tape removed after skinning drags the edge of the bead with it.

Keeping the seal working

The single maintenance instruction Selleys gives is to clean soap scum and dirt off the bead regularly. Those deposits are what mould feeds on, and they accumulate fastest at the base of a shower wall where water and residue collect. A bead that is wiped down as part of normal cleaning will outlast one that is not, regardless of the inhibitor in the silicone.

Ventilation does the other half of the work. A bathroom that dries out between uses will not sustain a colony.

Hazard classification

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

• Skin irritation — Category 2.
• Eye irritation — Category 2A. H319: causes serious eye irritation.
• Skin sensitisation — Category 1.

Not classified as Dangerous Goods under the Australian Code for the Transport of Dangerous Goods by Road & Rail or New Zealand NZS 5433.

What is in it

Chemical entity	CAS number	Proportion (w/w)
Silanetriol, ethyl-, triacetate	17689-77-9	1–10%
Acetoxysilane	4253-34-3	<1%
2-Octyl-2H-isothiazol-3-one (OIT)	26530-20-1	<0.5%
Non-hazardous or below reporting limits	—	Balance

Two ingredients explain most of this product's behaviour. The acetoxysilanes are the acetic cure system: they react with atmospheric moisture and release acetic acid, which is the vinegar smell over the first day and the reason this sealant must not go on copper, zinc, concrete, cement, mirror backing or stone.

OIT at under half a percent is the biocide that gives the product its mould resistance — and it is also the most likely reason for the skin sensitisation classification. Isothiazolinones are well recognised sensitisers, which is why a fungicidal sealant carries a classification that an ordinary one may not.

What skin sensitisation means. It is not the same as irritation. Irritation is a local reaction that settles once contact stops. Sensitisation is the immune system learning to react to the material — and once that has happened it is generally permanent, it can be set off by much smaller exposures than the one that caused it, and the reaction tends to worsen with each subsequent contact rather than fade. It typically develops after repeated casual contact, not one dramatic exposure, which is exactly the pattern of smoothing beads with a wet finger job after job.

Wear gloves, and tool the bead with a tool or a soapy finger inside a glove rather than bare skin. Once fully cured the sealant is inert; the hazard belongs to the uncured material.

Ventilate while it cures. Acetic acid vapour irritates eyes and airways in an enclosed bathroom, and a newly sealed shower recess is a small space with no airflow.

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 No Mould

How soon can I use the shower after applying it?

One hour after application. The Speed Seal formulation develops a waterproof skin in that time. Avoid touching the bead itself until it has fully cured.

How long does it take to cure completely?

At least 48 hours. The cure rate is 2mm per 24 hours, so a deeper bead takes longer. Shower-ready at one hour and fully cured are two different states.

Can I paint over it?

No. No Mould is not paintable. If the area needs painting, finish the painting seven days before you seal.

Will it stop mould permanently?

It resists mould growth, but the manufacturer conditions that claim on regular cleaning. Soap scum and dirt left on the surface will support mould regardless of the sealant underneath.

Can I use it in an aquarium or on a fish pond?

No. It is not suitable for aquariums, and not suitable for permanent water immersion.

Is it safe around food or drinking water?

No. It is not suitable for contact with food or drinking water. Choose a product rated for potable water contact for those applications.

Can I use it on metal?

Only aluminium and stainless steel. As an acetic cure silicone it is not suitable for other metals, which can corrode as the acid is released during cure.

Will it stick to plastic?

Not to PE, PP or PTFE. These plastics have too low a surface energy for silicone to bond to reliably.

Can I apply it over an existing bead of silicone?

No. Cut the old bead out completely first. New silicone does not bond well to cured silicone, and any mould in the old bead will grow through the new one.

What if I miss the 10-minute tooling window?

The surface starts skinning at around 20 minutes. Once that begins, tooling tears the bead rather than smoothing it, and the only fix is to cut it out and start again.

How do I remove cured No Mould?

Cut and scrape it out mechanically, or soften it first with a silicone remover, then scrape.

Which sizes does it come in?

A 290mL cartridge for a caulking gun, in transparent and white, and a 100mL tube for smaller jobs.