How to Repair Deep Cracks in Walls and Ceilings

Deep cracks in walls and ceilings are one of the most common jobs on any interior project. They happen: settlement, shrinkage, general wear and tear. The problem is when someone skims over them with the wrong product and they come straight back. A deep crack needs a filler that can handle the depth without shrinking or cracking, and a finish that makes the repair disappear.

What counts as a deep crack?

Generally speaking, a deep crack or hole is any crack that extends beyond 10mm. In fact, the width of the crack is a good measure of how much structural damage is present. For example, a 1mm-wide crack through a 10mm layer of plaster can be fixed easily, but a 10mm-wide crack would be considered more structural than decorative. For this article, though, we'll stick with the conventional measurement of depth, since that's what determines the right product to use. Powder products will still set at a depth beyond 10mm, while ready-mixed products rely on air drying, and that drying can be affected by the greater depth needed to fill beyond 10mm.

Step 1: Prep - clean out and stabilize

Widen the crack slightly with a scraper or utility knife to give the filler more surface area to grip. Clear out all the loose material, dust and debris. If there's old filler in there from a previous repair, get it out. You want clean, solid edges on both sides. Many tradespeople will cut back the edges to a 45-degree angle, which allows for a smoother transition and a larger surface area for better adhesion.

If you're unsure whether the surface needs stabilising, rub your hand or a dark cloth across the area. If it comes away dusty or powdery, or the surface crumbles under light pressure, the substrate is failing and needs treatment. Apply Wall Hardener liberally with a brush or roller, working it into the full area around the crack. Let it dry for a minimum of 24 hours before filling.

Step 2: Fill the crack

Pick the right filler for the job. Interior Filler is the all-round option. It fills to any depth in a single coat with no shrinkage and sets in 30-40 minutes. Quick Dry Filler does the same job but is ready to paint much sooner, in 3 hours. Quick Fill is a ready-mixed option if you'd rather skip the mixing altogether.

For powder fillers, add water to the tub first, then the powder. Mix to a smooth, lump-free consistency. To check it's right, load filler onto your filling knife and hold the knife vertically on its side. If the filler stays on the blade without sliding or dropping off, the consistency is correct. If it falls away, add more powder and mix again.

Using a filling knife, push the filler firmly into the crack. Get it right in, don't just spread it across the surface. These fillers handle any depth in one coat, so fill the whole crack in one pass. Aim for flush with the surrounding surface.

Once the filler is dry, sand back with medium-grit sandpaper (120 grit) to bring it level with the surrounding wall or ceiling.

At this point, the crack is repaired and the surface is ready for decoration.

Step 3: Finish - smooth for decoration

For a premium, paint-ready finish, we recommend one further step. Apply your chosen Toupret finish product over the repair and feather it out across the surrounding area so there's no visible edge where the repair starts and stops.

Let the finish coat dry completely, then lightly sand. The repair is now ready for painting or wallpapering. Toupret fillers won't flash or grin through paint, so there's no need to spot prime before you decorate.