Filling Cracks in Masonry Walls

How do you fill cracks in masonry walls and render?

Houses move. Foundations settle, clay soils swell and shrink with the seasons, and the result shows up as cracks in brickwork and render. Left open they let draughts in, and on an external wall they let water in behind the render. Filling them is a four-step job: clean the surface back to something solid, press the filler in firmly, let it dry, then sand and paint. The product Selleys specifies here is Selleys No More Cracks Large Cracks & Holes, a ready-to-use filler with superior adhesion to plaster, mortar, concrete, timber and primed metal.

Before you open the tub, though, there is one judgement to make, and it decides whether the repair lasts. Is the crack still moving? No More Cracks Large Cracks & Holes is a rigid filler. Its technical data sheet is explicit that where movement occurs you should use Selleys No More Gaps instead. A settled, stable crack takes filler and stays filled. A crack that is still opening will split the filler along the same line within a season, and you will be back with a scraper.

Telling a settled crack from a live one

Nothing in the Selleys guidance tells you how to make this call, and it is worth being honest that it is a judgement rather than a test. What the product documentation does tell you is the consequence: rigid fillers are for surfaces that do not move, flexible gap fillers are for joints that do. If a crack has reopened after a previous repair, or if it is wide, tapering and running diagonally across a wall, treat it as movement and use a flexible product — or get the wall looked at before you fill anything. Structural cracking is not a filler problem.

What the drying figures actually mean

Technology	Water-based acrylic
Appearance	Smooth paste
Sand / paint time — less than 2 mm	2–3 hours
Sand / paint time — up to 6 mm	24 hours
Adhesion	Plaster, mortar, concrete, timber, primed metal
Use	Interior and exterior
Shrinkage	Minimal

These are indicative figures only and are not intended for the preparation of specifications.

This is the part most people get wrong, so it is worth spelling out. The often-quoted "2–3 hours" applies to fills of less than 2 mm. Selleys' own instruction for deeper work is to build up in 6 mm layers, and the technical data sheet gives 24 hours as the drying time for a layer up to 6 mm. So a crack deep enough to need layering is a job spread over days, not an afternoon. Sanding or overcoating a 6 mm layer at three hours will tear the soft filler out and leave you worse off than when you started.

The minimal shrinkage claim is what makes this workable. Because the filler barely shrinks, you do not have to overfill each layer to compensate; you fill flush, let it dry, and add the next. Overfill only the final layer, and only slightly, to minimise the sanding.

What you need

• Selleys No More Cracks Large Cracks & Holes
• A filling blade or scraper
• Fine sandpaper
• Selleys Sugar Soap
• Water-based undercoat and a water-based topcoat

How to fill the crack

1. Prepare the surface. Make sure the surface to be filled is clean, dry and free of loose or powdery material. On render this often means brushing or scraping until you reach sound material — filler applied over crumbling render is only as strong as the crumbs underneath it. It is also recommended that exterior surfaces be primed before filling.
2. Press the filler in. Use a scraper or filling blade to firmly press Selleys No More Cracks Large Cracks & Holes into the crack, and scrape off any excess as you go to save time sanding later.
3. Build up deep cracks in layers. For deeper jobs, apply the filler in 6 mm layers, allowing each layer to dry before the next. Allow 24 hours per 6 mm layer; a thin skim of less than 2 mm needs 2–3 hours. Overfill only the final layer, and only slightly.
4. Let it dry. Humidity extends all of these times. Do not use the product below 5°C.
5. Sand back. Sand lightly with fine sandpaper once the fill is completely dry.
6. Clean the surface. Clean the whole area with Selleys Sugar Soap. A clean surface aids paint adhesion, and after sanding masonry there is a great deal of dust.
7. Undercoat and paint. Apply a water-based undercoat for best results, then paint over with a water-based paint. Outside, protect the repair from rain until it has been painted over.

Limitations — where this filler should not be used

• Do not use below 5°C. Cold slows the cure and can stop it setting properly at all.
• Not suitable for exterior timber likely to absorb excessive amounts of water. Timber that takes up water swells and shrinks; a rigid filler cannot follow it.
• Where movement occurs, use Selleys No More Gaps. This is the important one for masonry. A crack caused by ongoing settlement needs a flexible product, not a rigid filler.
• Protect exterior work from rain until it has been painted over.

Clean up and storage

Uncured product comes off with water. Once cured, it has to be removed by sanding and scraping, so wipe your blade and your hands before the filler sets. Store the tub in a cool, dry place with the lid closed.

Safety: the eye hazard sits in step 6, not in the filler

Two products in this method carry a classification, and the more serious one is the cleaner.

Selleys Sugar Soap is classified Danger — H318, causes serious eye damage (serious eye damage, Category 1). That classification belongs to the concentrate in the bottle rather than to the half-cup-in-five-litres mix you wash a wall with, and the difference is what makes the precaution practical. Category 1 is not irritation that settles: the concentrate's safety data sheet describes corneal burns, and the response is fifteen minutes of irrigation with the eyelids held open and urgent medical attention — not a rinse at the tap and carrying on.

Step 6 is where that bites. Sanding masonry and render leaves the wall coated in dust, so this is the job where people reach for the neat product and a scouring sponge on stubborn in-ground marks, which the directions allow. That, and measuring and pouring the concentrate into the bucket, are the two moments you are handling the undiluted material. Have eye protection on for both.

The external-wall geometry adds the second problem. Filling render usually means reaching above head height or working off a ladder with a wet cloth, which is precisely the arrangement that delivers cleaner to an upturned face. Wash from the top down, keep the bucket below you rather than on the step above, and do not carry the concentrate up the ladder to top the bucket up — mix at ground level.

No More Cracks Large Cracks & Holes carries Warning — H317, may cause an allergic skin reaction. That is skin sensitisation, not irritation. It is acquired by repeated casual contact and is generally permanent once acquired, after which much smaller exposures set it off. Pressing filler firmly into a crack with a bare thumb, along a whole wall, and again next season, is exactly that pattern of exposure. Use the blade, and glove up for the parts a blade cannot reach.

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

Common questions about filling cracks in masonry walls

How long does the filler take to dry?

2–3 hours for a fill less than 2 mm deep, and 24 hours for a layer up to 6 mm. Humidity lengthens both.

Can I fill a deep crack in one pass?

No. Selleys directs you to build up in 6 mm layers, letting each layer dry before applying the next.

Do I need to overfill?

Not for shrinkage — the filler has minimal shrinkage. Overfill the final layer only slightly, so that sanding levels it rather than cutting into the wall.

Can I use this outside?

Yes, it is rated for interior and exterior use. Prime exterior surfaces before filling and keep the repair out of the rain until it is painted.

What surfaces will it stick to?

It has superior adhesion to plaster, mortar, concrete, timber and primed metal.

The crack came back. What now?

That is the signature of a moving joint. Selleys' instruction where movement occurs is to use No More Gaps rather than a rigid filler. If the crack is wide, growing or structural in character, have the wall assessed before filling it again.

Can I paint over it?

Yes. Clean the sanded surface with Sugar Soap, apply a water-based undercoat for best results, then a water-based topcoat.

Is there a temperature I should not work below?

Do not use the product below 5°C.

How do I get the filler off my tools?

Water, while it is still uncured. Cured product needs sanding or scraping.

How long should the whole job take?

A shallow crack is a couple of hours of drying plus sanding and painting. A deep one built up in 6 mm layers is a day per layer, which is worth planning for before you start.

Should I fill cracks before or after painting the wall?

Before. Fill, dry, sand, clean with Sugar Soap, undercoat, then paint.