Selleys Ask an Expert: Tips for Using Rigid Fillers

What are rigid fillers, and when do you need one instead of a flexible filler?

A rigid filler sets hard. That is the whole distinction. Where a flexible filler stays elastic so it can move with the building, a rigid filler cures into a solid you can sand flat, shape, and in some cases drill and screw into. The questions in this section are mostly variations on one theme: which Selleys filler will actually hold something once it has set?

The answer depends on what you are going to do to the filled area afterwards, and there are three broad tiers in the Selleys range.

Tier 1 — Surface fillers, for holes you are only going to paint

Spakfilla Rapid and Spakfilla Heavy Duty are ready-mixed lightweight fillers for wallboard, wood, masonry, plaster and primed nail heads. They are fast drying, virtually non-shrinking and easy to sand.

What they cannot do is carry anything. Both carry the same limitations: do not nail into the dried filler, and not for load bearing applications — the example given is behind a door where the knob touches the wall, where the filler would simply be crushed. They are also not for use on plastic or metal, or on exterior timber likely to swell and shrink with water, and they should not be applied below 5°C or in wet conditions.

Tier 2 — Epoxy putties, for repairs that need to hold a fixing

Selleys Knead It Multipurpose is a hand-kneadable, fast-setting epoxy that bonds to timber, metal, masonry, plastics and ceramics. Its listed uses include anchoring bolts, screws and housings, filling rust holes and small dents in metal, and moulding to reform damaged timber and metal.

Its published figures: working time 3 to 5 minutes at 25°C, initial cure 60 minutes, full cure 24 hours at 25°C, Shore D hardness 80±5 (ASTM D2240). After 60 minutes it can be drilled, sanded, machined, filed, sawed and painted. Set time increases below 20°C.

Tier 3 — Two-pack bogs, for filling that will be drilled and screwed

Selleys Plasti-Bond Heavy Duty and Selleys Tradie’s Bog are two-pack fillers — a resin and a hardener you mix immediately before use. Once cured they can be sanded, drilled and screwed into. This is the tier to reach for when a bracket, a hinge or a fitting has to be re-fixed into a hole that has been filled.

The mistake this section exists to prevent

Using a flexible filler where a rigid one is needed. Selleys No More Gaps is highly flexible and does not cure hard, which is exactly why it cannot be drilled or screwed into. It is the right product for a moving joint and the wrong product for a fixing point.

The reverse mistake is just as common. Every Spakfilla data sheet carries the instruction that where movement occurs, use Selleys No More Gaps — because a rigid filler in a moving joint will crack straight down the line.

Questions answered in this section

• Which filler can be drilled and screwed into?
• Filling screw holes so a screw can go back in
• Filling splits and holes in a timber slab
• Filling a gap in a granite benchtop join
• Colouring Plasti-Bond or Tradie’s Bog

Safety: the three tiers carry three different classifications

The tiers above are sorted by what the filler can hold. They also sort, almost exactly, by hazard — and the jump happens at Tier 3.

Product	Classification
Spakfilla Heavy Duty	Not classified as hazardous — no signal word, no hazard statements
Spakfilla Rapid	Warning — H317 may cause an allergic skin reaction, H319 causes serious eye irritation
Knead It Multipurpose	Warning — skin sensitisation Category 1A, skin irritation Category 2, eye irritation Category 2A
Tradie’s Bog	Danger — Part A: H361 suspected of damaging fertility or the unborn child, H372 damage to organs through prolonged or repeated exposure, H226, H315, H317, H319, H335. Part B hardener: organic peroxide Type E, H242 heating may cause a fire, H317, H319

H361 is the reason Tier 3 is not a casual choice. Styrene is 30 to 60% of Part A — it is the bulk of the filler and the sharp smell — and it is a suspected reproductive toxicant. That makes Tradie’s Bog a product to avoid entirely if you are pregnant or trying to conceive, and one that needs real ventilation in any case: not a garage with the door half open.

H372 turns on the word "repeated". One bracket refixed in a bathroom is a different exposure from a tradesman mixing bog weekly. If this is a regular job rather than a one-off, that statement is the one aimed at you.

H242 on the hardener is a storage instruction. Dibenzoyl peroxide can decompose exothermically if it gets hot, so keep the hardener tube out of sunlight and out of a hot vehicle — and do not mix more than you need, because a large catalysed mass generates its own heat as it cures.

Both parts carry H317, and so does Knead It at the stronger 1A sub-category. Sensitisation is not irritation: permanent once acquired, then triggered by much smaller exposures. Knead It's format invites it — a putty designed to be kneaded by hand mixes most easily with bare fingers, and doing that repeatedly is the classic route to epoxy sensitisation. Wear gloves for mixing and application, every time.

And the shared hazard of this whole section is dust. The premise of a rigid filler is that it gets sanded. Sanding any of these produces respirable dust, including the unclassified one. Dust mask, eye protection, and wet sanding or extraction where practical.

Choosing down the list where the job allows it: if the hole is only going to be painted, Spakfilla Heavy Duty carries no classification at all. If it has to hold a fixing, Knead It gets there without H361 or H372 attached. Reserve the two-pack bogs for what only they can do.

Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand.

Common questions about rigid fillers

Can I drill into Spakfilla?

It is not designed for it, and its data sheet says not to nail into the dried filler and not to use it in load bearing applications. For a hole that has to take a fixing, use Knead It or a two-pack bog.

Which Selleys fillers can be drilled and screwed?

Knead It Multipurpose, after its 60 minute initial cure, and the two-pack bogs Plasti-Bond Heavy Duty and Tradie’s Bog once cured.

How long before I can sand a filler?

Spakfilla Rapid: about 30 minutes for applications under 2mm, up to 24 hours over 2mm. Spakfilla Heavy Duty: 2 to 3 hours for a 2mm application. Knead It: 60 minutes.

Can I fill a deep hole in one go?

With Spakfilla Heavy Duty, build up deeper cracks in 6mm layers and let each layer dry.

Can I use a rigid filler outside?

Spakfilla Heavy Duty can be used on exterior surfaces, but exterior applications must be protected from rain until fully dry and then painted with two coats.

What if the crack keeps reopening?

That is movement, and no rigid filler will hold it. Use No More Gaps instead.

Can I paint a rigid filler?

Yes. Spakfilla Rapid takes water-based paint 30 minutes after a thin application; allow 24 hours for oil-based paints. Knead It can be painted after 60 minutes.

Are the two-pack bogs hazardous?

Both Plasti-Bond Heavy Duty and Tradie’s Bog are classified as hazardous and are supplied as a resin plus a separate hardener. Read the label and the safety data sheet before mixing.