Which Selleys specialist filler or putty do you need, and how do you choose?

This is the part of the Selleys filler range that exists for the jobs a general filler cannot do. The flexible and rigid fillers elsewhere in putty and fillers are for gaps, cracks and holes on most surfaces. The products here are chosen by what you are repairing and what it has to survive — metal, water, heat, a car panel — rather than by the size or shape of the defect.

The single most useful way to navigate the range is to notice that most of it is the Knead It family: hand-mouldable putties, each formulated for a different material or environment. If your repair is on something other than a flat wall, that is where to start.

What this category covers

Selleys makes the general point on this category page that its flexible and rigid fillers are good for filling gaps, cracks and holes on most surfaces — but that when it comes to working with timber and glazing, a specialist putty is what you need. Two products it names for that work, Selleys Special Putty and Selleys Woodfilling Putty, are catalogued under wood putty rather than in the listing below. Their key restriction is stated on this page and is worth repeating: both are developed for use with timbers and should not be used on metal window frames.

The products listed under this path itself are the epoxy putties and purpose-built compounds described below.

How to choose

If you are repairing metal

Knead It Steel is a fast-setting, non-rusting co-extruded epoxy repair system that can be used for metal items. Two of those words matter more than the rest. Non-rusting means the repair itself will not become the corrosion site — which is the usual failure of a metal patch made with the wrong material. Co-extruded describes the two-part format: the resin and the hardener come together and are combined by kneading, which is what triggers the set.

If the repair is wet, or will be

Knead It Aqua is the waterproof, mouldable option, described for plumbing and underwater repairs. It is the one to use when you cannot drain and dry the thing you are fixing, which on a live plumbing system is most of the time.

If you are repairing a vehicle

Knead It Autofix is described as a professional-grade solution for car repairs. Use the automotive product for automotive work rather than a general putty — panel and under-bonnet environments differ from household ones in temperature and vibration.

If the repair is general and you are not sure

Knead It Multipurpose is the general-purpose member of the family, described for DIY projects and repairs meant to last. Where none of the specific formulations matches your material, this is the default.

If the repair is near fire or heat

Selleys Fireproof Cement is described for high-risk areas such as fireplaces and chimneys. Nothing else in this range is described for that environment, so do not substitute a general putty into a fireplace or flue repair.

If you need to mould the filler by hand

Selleys Plasti-Bond Heavy Duty is a plastic putty for repairing plastic, metal and other surfaces, and it can be shaped before it hardens. Reach for it when the repair has a form to it — a broken edge, a missing corner — rather than a flat void to fill and sand.

If you are making fast repairs on a building site

Selleys Tradie's Bog is a building filler that is easy to sand and gives fast repairs to timber and most common building materials. Easy sanding is the property that decides whether a filler suits site work: a hard-setting compound that fights the sanding block costs more time than it saved in setting quickly.

If you are sealing rather than filling

Selleys N-Mastic sits in this listing but belongs to the flexible gap filler family — it is described as giving a strong and flexible bond for sealing gaps and joints in surfaces. If your job is a joint rather than a repair, compare it against the products in gap filler.

The products in this range

• Knead It Steel — fast-setting, non-rusting co-extruded epoxy repair system for metal items.
• Knead It Aqua — waterproof and mouldable, for plumbing and underwater repairs.
• Knead It Autofix — professional-grade putty for car repairs.
• Knead It Multipurpose — versatile and durable, for general DIY projects and repairs.
• Fireproof Cement — for high-risk areas such as fireplaces and chimneys.
• Plasti-Bond Heavy Duty — plastic putty for repairing plastic, metal and other surfaces, mouldable before it hardens.
• Tradie's Bog — building filler, easy to sand, for fast repairs to timber and most common building materials.
• N-Mastic — flexible gap filler for sealing gaps and joints in surfaces.

The safety divide in this category — the polyester fillers are not like the rest

The page above groups these by the repair you are making. Grouped by hazard instead, they fall into three clear bands, and the band a product sits in follows its chemistry rather than how hard the job looks.

Group	Products	Classification
Danger — polyester fillers	Tradie's Bog, Plasti Bond Heavy Duty	Tradie's Bog Part A: H361 (suspected of damaging fertility or the unborn child), H372 (organ damage from repeated exposure), H226, H315, H317, H319, H335. Both hardeners: H242 (organic peroxide, Type E — heating may cause a fire), H317, H319
Danger — alkaline cement	Fireproof Cement	Eye damage Category 1 (causes serious eye damage), H315, plus respiratory tract irritation Category 3
Warning — the epoxy putties	Knead It Multipurpose, Knead It Steel, Knead It Autofix	H317 skin sensitisation (Category 1A for Multipurpose and Steel, Category 1 for Autofix), H315, H319. Multipurpose also carries a chronic aquatic hazard. None are Dangerous Goods
No classification published	Knead It Aqua, N-Mastic	No hazard classification is published for either product in the material available here — see product page. This is an absence of data, not a statement that they are unclassified

The single most important line on this page is H361 on Tradie's Bog, and it applies to no other product in the category. Part A is 30–60% styrene, which is both the sharp smell and a suspected reproductive toxicant — suspected of damaging fertility or the unborn child. If you are pregnant, trying to conceive, or sharing a workshop with someone who is, that is a reason to choose a different filler rather than a reason to open a window. Where a repair could be made with an epoxy putty instead, the epoxies carry no reproductive classification at all.

Both polyester hardeners are organic peroxides, and that is a storage rule as much as a handling one. H242 — heating may cause a fire — describes dibenzoyl peroxide decomposing exothermically when it gets hot. Do not leave the hardener tube in a hot vehicle, on a sunlit bench or beside a heater, and store it separately from the filler. The same chemistry is why pot life collapses with batch size: a large catalysed mass generates its own heat, which accelerates the reaction further. Mix only what you can place.

Across the three Knead It products the classification to weigh is H317, not H315. Irritation is local and reversible; sensitisation is the immune system learning to react, generally permanent once acquired, and triggered afterwards by far smaller exposures. Multipurpose and Steel are Category 1A — the sub-category used where there is strong evidence of sensitising humans, a step more definite than plain Category 1. This matters more for this format than any other, because a hand-kneadable putty invites bare fingers: streaky putty is weak putty, and the easiest way to get a uniform mix is with your hands. That is the exact habit that acquires a permanent epoxy allergy. Gloves, every time.

Fireproof Cement is a Danger product that looks like grey paste. Its eye classification is Category 1 because sodium silicate solution — water glass, 10–30% of the tub — is strongly alkaline, and alkali eye injuries keep penetrating tissue rather than self-limiting the way acid burns do. Read that against how the product is actually used: kneeling with your head inside a firebox, applying upward. Eye protection is not optional in that position. Once set and fired the cement is inert; the whole hazard belongs to the wet material.

Two products publish no classification here. For Knead It Aqua that is notable, because its three Knead It siblings all carry Category 1 or 1A sensitisation and it is the same family of chemistry — treat the absence as missing data and glove up anyway rather than reading it as an all-clear. N-Mastic publishes a full technical data sheet but no safety classification.

Comparing the fillers on the numbers that decide the job

For a repair putty, three figures decide everything: how long you can shape it, how soon you can work it, and how hot it can get afterwards.

Product	Working time	Handling / machinable	Full cure	Heat resistance
Tradie's Bog	~5 minutes at 23°C	~20 minutes — saw, drill, sand, paint	See product page	See product page
Plasti Bond Heavy Duty	~5 minutes at 23°C	~20 minutes — saw, drill, sand, paint	See product page	See product page
Knead It Steel	3–5 minutes at 25°C	60 minutes (handling only)	24 hours at 25°C	See product page
Knead It Autofix	3–5 minutes at 25°C	60 minutes (handling only)	24 hours at 25°C	−40°C to +120°C continuous; +140°C intermittent
Knead It Multipurpose	~3–5 minutes	~60 minutes — drill or sand	~24 hours	Up to 120°C continuous
Knead It Aqua	~20–30 minutes	~60 minutes (set)	~24 hours	120°C continuous
Fireproof Cement	See product page	See product page	48 hours minimum before heat	Designed for stoves and fireplaces — see product page
N-Mastic	Never sets — remains permanently plastic			−40°C to +80°C

The fastest products to a finished repair are the ones with the worst classifications, and the gap is large. A polyester filler is sawable, drillable, sandable and paintable in about 20 minutes. An epoxy putty needs 24 hours before it reaches the same state — the 60-minute figure on the Knead It products is a handling milestone, not permission to machine. That is a 70-fold difference in turnaround, and it is precisely why the polyester products exist. It is also why the styrene and the organic peroxide are in them. If you can afford to come back tomorrow, the epoxy route costs you nothing but time and buys you out of the two most serious classifications in the category.

Knead It Aqua is the outlier inside its own family, by a factor of six. Its siblings give you 3–5 minutes of working time; Aqua gives you 20–30. On a repair that is by definition awkward — a wet pipe, a tank, something you are reaching around behind — the difference between four minutes and twenty-five is the difference between a rushed smear and a properly formed repair. If the constraint on a job is access rather than water, Aqua is still the right choice for the working time alone.

Five minutes is a real constraint, and it does not scale with batch size. Both polyester fillers and three of the four Knead It products give you roughly five minutes once the parts are combined, so the surface must be prepared and the spatula in hand before you mix, and one batch equals one repair. Mixing a larger batch to cover several repairs does not extend the window — with the polyesters it shortens it, because the mass heats itself.

Two temperature notes. The epoxy putties cluster at 120°C continuous, which rules them out for exhaust components and anything sitting against a manifold — that is a Fireproof Cement job, and Fireproof Cement needs its 48 hours of drying before heat goes anywhere near it. And N-Mastic is the only product here that never hardens at all: the seal stays permanently plastic, which is the point of it, and also why it will not resist being disturbed.

Common questions

What is the difference between the Knead It products?

They are formulated for different materials and environments: Steel for metal items, Aqua for plumbing and underwater repairs, Autofix for car repairs, and Multipurpose for general DIY work.

Which one should I use on a leaking pipe?

Knead It Aqua, which is the waterproof, mouldable option described for plumbing and underwater repairs. Check its product page for the conditions and cure time before relying on it in a live system.

Will a Knead It repair on metal rust?

Knead It Steel is described as a non-rusting epoxy repair system for metal items, which is the point of using it rather than a general filler on metal.

What should I use around a fireplace?

Fireproof Cement is the product described for high-risk areas such as fireplaces and chimneys. Do not substitute a general filler or putty into that application.

Can I use these putties on timber window frames?

For timber and glazing work Selleys points to its specialist putties — Special Putty for fitting windows into wooden frames and joinery, and Woodfilling Putty for nail holes, cracks and minor flaws in timber. Both are listed under wood putty.

Can I use a wood putty on a metal window frame?

No. Selleys states plainly that both specialist putties are developed for use with timbers and should not be used on metal window frames.

Which product can I shape by hand before it sets?

Plasti-Bond Heavy Duty is a plastic putty that can be moulded before it hardens. The Knead It products are also hand-worked, since kneading is what combines and activates them.

Is Tradie's Bog only for timber?

It is described as giving fast repairs to timber and most common building materials, and as being easy to sand.