How to Fill a Wall Crack: Filler and Tape, the Pro Method

A crack on a wall is as unsightly as it is worrying, but in the majority of cases it fills easily and durably. The key to success does not lie in the quantity of filler, but in the method: prepare the substrate, choose the right product and reinforce the crack with tape to stop it reappearing. Here is the complete guide, from diagnosis to the final sanding.

Assessing the crack before filling it

Not all cracks are treated the same way. A fine, stable crack (less than 1 mm) is simply filled. A wider crack or one in a wall corner, above a door or at the junction of two materials can signal structural movement. If it exceeds 3 mm, if it runs diagonally down the full height of the wall or if it comes with an offset of the edges, have it checked by a professional before any repair. For the rest, you can do it yourself.

The materials you need

• Filler or smoothing compound for cracks up to 2 mm
• Paper or fibreglass joint tape (adhesive mesh) to reinforce the crack
• Wide filling knife (20 cm) and medium spatula (10 cm)
• Abrasive paper grit 120 and 180, sanding block
• Brush, damp cloth and sometimes a little degreaser
• Primer if the surface is smooth or already painted

For fine cracks, prefer a micro-perforated reinforced paper tape, ideal on drywall, or an adhesive fibreglass tape for cracks filled flush. The tape acts as reinforcement: it absorbs the micro-movements of the substrate and stops the crack reopening at paint level.

Preparing the substrate and opening up the crack

1. Widen the crack slightly with the point of a knife or a small spatula to create a V-shaped groove (undercut) where the filler will grip
2. Brush the inside to remove all the dust and fragments
3. Run a slightly damp cloth over the edges to pick up any remaining dust
4. Let dry completely (30 minutes to 1 hour) before applying the filler

Opening up the crack is the step most DIYers skip, and it is the main cause of cracks that come back. By creating a slight V-shaped widening, you give the filler a mechanical grip: a crack that is too fine, simply filled, forms a thin layer that crazes and re-cracks at the first movement of the wall.

Applying the filler then laying the tape

1. Apply a first pass of filler into the crack with the spatula, pressing well to make it penetrate
2. Immediately lay the tape over it, still wet, and press it down with the spatula to bed it without bubbles
3. Cover the tape with a thin coat of filler overlapping 5 cm on each side, smoothing from the outside towards the centre
4. Let dry for 12 to 24 hours
5. Apply a second, wider pass of filler, overlapping 15 to 20 cm, to blend the area into the wall, then sand

Sanding and finishing the repaired area

Sand first with grit 120 to remove the excess filler, then with grit 180 for a smooth surface. Always sand with crossing strokes to avoid hollows, and check the flatness with the flat of your hand or a straightedge: the filler must come level with the wall, neither hollow nor raised. Then dust off with a brush and a damp cloth.

• Sand with grit 120 then 180 over an area 30 cm wide around the crack
• Systematic dusting before painting, otherwise the paint adheres poorly
• Applying a primer over smooth sanded areas or old satin paints
• Two coats of finishing paint to even out the shade with the rest of the wall

The mistakes to avoid

• Filling without opening up or dusting the crack: the filler does not hold and the crack reappears within weeks
• Skipping the reinforcing tape on a crack in a corner or above an opening
• Applying a coat of filler that is too thick: it dries badly and shrinks
• Painting directly onto the filler without a primer, which gives visible matt patches
• Sanding too early, before complete drying: the surface marks and clogs

Special cases: cracks in the ceiling and on plasterboard

On a plasterboard partition, a fine vertical crack on a joint often signals a badly made joint: you then have to shave the joint, lay a reinforced paper tape and fill in two passes. On a ceiling, the crack generally follows a joint between sheets: proceed the same way working on a stable step ladder, and beware of cracks at joinery corners, which always come back if you do not reinforce them.

With these steps — diagnosis, opening up the crack, filler reinforced with tape, two passes and careful sanding — your wall stays clean and smooth for years. Allow one to two hours of work for a 50 cm crack, plus the drying time, and you get an invisible result once the paint is applied.