How To Fill a Gap Between a Cornice and Wall

What product should you use to fill the gap between a cornice and a wall?

Use Selleys No More Gaps Multipurpose. It has the flexibility to cope with building movement, which is what keeps the gap or crack hidden for longer rather than reopening a few months after you have painted.

That flexibility is the whole reason a gap filler beats a wall filler here. The cornice-to-wall junction is a joint between two elements that are not rigidly bonded to each other, and a house frame moves — seasonally, and as it settles. A hard-setting plaster filler in that line will crack straight down the middle. No More Gaps Interior Multipurpose is rated at up to 15% movement capability (ISO 9046) with elongation at break over 300% (ASTM D412), so it stretches instead of splitting.

How to apply it

1. Make sure the surface is clean, dry and free of dust.
2. If you want the neatest possible edges, run masking tape along both sides of the gap first. This is more worthwhile on a cornice than almost anywhere else in the house, because the line is at eye level from across the room and any wobble shows.
3. Apply a small bead of product along the entire gap.
4. Wipe the residue off with your finger, or with the back of a teaspoon dipped in a small amount of detergent, to leave a smooth finish. The detergent stops the filler dragging on whatever you are tooling with.
5. Remove the masking tape before the product skins — about 20 minutes — while the edges will still flow together rather than tearing.
6. Clean up any excess with a damp cloth immediately, while it is still wet.

When you can paint

This is the part that determines whether the finished cornice looks right, and a cornice is almost always painted with the sort of paint that needs the longest wait. The published paint-over times for No More Gaps Interior Multipurpose are:

Semi-gloss	0 minutes
Gloss	10 minutes
Low sheen	10 minutes
Flat	100 minutes
Ceiling	120 minutes

Ceiling paint needs two hours, and flat paint not much less. The reason is that both are highly filled and contain less binder than a gloss or semi-gloss, so the dried film flexes less and is far more likely to craze as the filler underneath finishes drying. Rushing this step is the single most common cause of a hairline crack appearing along a freshly painted cornice line.

For oil-based paints, allow 24 to 48 hours.

The published figures

Technology	Water-based acrylic
Colour	White
Density	1.64
Skin time	Approximately 20 minutes (23°C, 50% RH)
Cure to touch	30 minutes before painting or sanding
Full cure	24 hours
Movement capability	Up to 15% (ISO 9046)
Elongation at break	+300% (ASTM D412)
Maximum gap	10mm wide × 5mm deep
Shelf life, unopened	24 months

Indicative data only, not intended for the preparation of specifications.

Gap size, and why the depth limit matters

The maximum recommended gap is 10mm wide by 5mm deep. Cornice gaps are often deeper than they look, because the cornice sits proud of the wall and there is a void behind it. Do not fill that void. A deep bead of a water-based product shrinks more as the water leaves it, and shrinkage is what cracks the paint film over the top.

Fill to no more than 5mm of depth. If the gap is genuinely large, build it up in passes, letting each one dry, or use a product intended for bigger voids such as No More Big Gaps.

Limitations

• Apply above 10°C. Cold or humid conditions extend drying times considerably.
• Not suitable for permanent water immersion or constantly damp areas.
• Not suitable for gaps between tongue and groove boards or weatherboards — use No More Gaps Exterior for those.
• Not suitable for polyethylene, polypropylene or Teflon.
• Avoid applying paint too thickly over the bead.

Safety: the void behind the cornice, and what not to fill it with

The product this page recommends is the mild one. No More Gaps Interior Multipurpose is not classified as hazardous under Safe Work Australia GHS 7 — no signal word, no hazard statements, and not Dangerous Goods. That is a definite finding from its own safety data sheet rather than missing information, and it does not hold across the rest of the No More Gaps family, so it is worth knowing. It is also why smoothing the bead with a finger, as step 4 describes, is reasonable here in a way it is not with most sealants.

The alternative this page names for a genuinely large gap is a different proposition entirely. No More Big Gaps is an isocyanate polyurethane foam classified Danger, carrying H222 (extremely flammable aerosol), H334 (may cause allergy or asthma symptoms or breathing difficulties if inhaled), H351 (suspected of causing cancer — the lower of the two carcinogenicity statements), H372 (causes damage to organs through prolonged or repeated exposure) and H362 (may cause harm to breast-fed children).

A cornice line is close to the worst place in the house to use it. The work is directly overhead, so your face is under the nozzle. The void behind the cornice is enclosed, which is where the expanding foam and its vapour go. And a room being prepared for painting is usually shut up. H334 is respiratory sensitisation: the immune system learns to react to isocyanate vapour, the reaction is generally permanent once acquired, it is then triggered by concentrations far below the one that caused it, and it worsens with each exposure. The exposure window is from the moment you press the trigger until the foam has fully cured — cured foam is inert.

If you do use it: an organic vapour respirator rather than a dust mask, gloves, eye protection, active ventilation rather than an open door, and nobody else in the room until it has cured and the space has been aired — particularly children and anyone with asthma. Anyone breastfeeding should not use it at all. Do not pierce or burn the can even when empty.

The route this page already recommends avoids all of that. Do not fill the void. Fill to no more than 5mm of depth, build up in passes if you need to, and stay on a product that carries no classification. A deep cornice gap is a reason to work in layers, not a reason to foam it.

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

Common questions about cornice gaps

Why not use a wall filler like Spakfilla?

Because the cornice line moves. Selleys directs you to No More Gaps wherever movement occurs — Spakfilla Rapid and the other rigid fillers carry that instruction on their own data sheets.

How long before I can paint?

Ceiling paint needs 120 minutes and flat paint 100 minutes. Gloss and low sheen need only 10 minutes, and semi-gloss can go on as soon as the filler has skinned. Oil-based paints need 24 to 48 hours.

How do I get a clean edge?

Mask both sides before you apply, tool the bead, then pull the tape off before the product skins at around 20 minutes.

What can I use to smooth the bead?

A finger or the back of a teaspoon, dipped in a small amount of detergent. The detergent stops the product dragging.

How big a gap can I fill?

Up to 10mm wide and 5mm deep. Deeper than that, build it up in layers.

How long until it is fully cured?

24 hours for full cure, with cure to touch at around 30 minutes.

Can I sand it?

Yes, once it has cured to touch at about 30 minutes — though on a cornice a well-tooled bead usually needs no sanding at all.

What if the crack comes back?

A recurring crack in the same line usually means either the bead was too deep and shrank, or the paint went on before the published paint-over time. Cut it out, refill to 5mm maximum depth, and observe the wait for your paint type.

Is there a guide to installing cornices?

Yes — how to install cornices covers the job from the start.