Hairline cracks and settlement cracks are a fact of life in UK homes. Most appear in plaster over timber-framed partitions, around door and window frames, or at ceiling-wall junctions — none of which are cause for structural alarm on their own. The problem comes when they are painted over without proper filling, creating visible ridges that re-open within months. Done correctly, filling cracks before painting takes no more than an afternoon of work and produces a flat, durable surface.
The critical distinction is between cosmetic cracks (typically less than 2 mm wide, surface only) and structural cracks (wider than 3–5 mm, diagonal, or progressive). Structural cracks need professional assessment before any filling work begins. This guide covers cosmetic and settlement cracks only.
Tools and Materials
- Filling knife (5-inch and 3-inch blades)
- Stiff brush or vacuum for loose debris
- PVA adhesive (unibond or similar)
- Decorator’s flexible filler or fine surface filler
- Multi-finish plaster (for deeper repairs over 5 mm)
- Jointing tape or fibreglass mesh tape (for cracks longer than 200 mm or at movement joints)
- Fine-grit sandpaper (120 grit and 180 grit)
- Sanding block
- Mist coat primer or diluted emulsion
Understanding the Crack Types
| Crack Type | Width | Likely Cause | Correct Fix |
|---|---|---|---|
| Hairline / spider cracks | < 1 mm | Plaster shrinkage or ageing | Fine surface filler |
| Settlement cracks | 1–3 mm | Building movement / seasonal timber | Flexible decorator’s filler |
| Plasterboard joint cracks | 1–5 mm | Framing movement or poor taping | Jointing compound + mesh tape |
| Deeper structural defects | > 5 mm | Subsidence, lintel failure | Professional investigation first |
Step 1 — Diagnose and Monitor
Before filling, check whether the crack is active (still moving) or stable. Use a pencil to mark the ends of the crack and date it. Monitor for four weeks. Active cracks may be seasonal — normal in timber-framed partitions as joists expand and contract — but they need a flexible filler rather than a rigid one, or they will re-crack.
Step 2 — Open and Clean the Crack
Dragging a filling knife down the crack to widen it slightly sounds counterintuitive, but a V-shaped profile gives the filler a mechanical key and prevents a thin skin of filler from popping out later. Remove all loose plaster, dust, and debris using a stiff brush or the nozzle of a vacuum cleaner. Blow any remaining dust out of deep cracks with a puff of breath.
Step 3 — Prime with PVA
Dilute PVA adhesive 3:1 with water and brush it into the crack and onto the surrounding surface. This seals the suction of the bare plaster, stops the filler drying too fast (which causes shrinkage), and improves adhesion. Allow the PVA to become tacky — roughly 15–20 minutes — before filling.
For cracks longer than 200 mm or at joints between different substrates (plaster to timber, plaster to brick), bed a strip of fibreglass mesh tape or paper jointing tape into a thin skim of filler first. This bridges the joint and prevents the crack simply breaking through the filler.
Step 4 — Apply the Filler
Hairline cracks: A single pass of fine surface filler (products such as Polycell Fine Surface Filler or Ronseal Smooth Finish Filler, £4–£10 per 500 g tub) applied with a flexible 3-inch knife is usually sufficient. Press the filler firmly into the crack, drawing the knife at 45° to the surface to pack it, then flatten off in a single sweep. Leave slightly proud — it will shrink by 5–10% as it dries.
Medium cracks (1–5 mm): Use a decorator’s flexible filler for any crack subject to movement. Apply in a single pass for cracks under 3 mm; for wider cracks, apply a first coat, allow to dry fully (24 hours), then apply a second coat. Do not try to fill a wide crack in a single thick application — it will crack again as it dries.
Deep voids (over 5 mm): Pack the void with a scrunched piece of fibreglass mesh or damp newspaper to within 5 mm of the surface, then build up with bonding plaster or multi-finish plaster. Finish the final 1–2 mm with a fine surface filler or a skim of multi-finish.
Step 5 — Sand Back
Once the filler is completely dry — typically 24 hours at room temperature, longer in cold or damp conditions — sand the repair flat using 120-grit paper on a sanding block. Work in a circular motion, then finish with 180 grit along the length. The aim is a surface that is flush with the surrounding plaster and, importantly, matt (gloss from fine-surface fillers must be cut back or it will show as a sheen through emulsion).
Wipe the sanded area with a slightly damp cloth to remove dust and re-seal if necessary.
Step 6 — Apply a Mist Coat Before the Topcoat
Bare filler and exposed plaster are highly absorbent. Applying a full-strength emulsion over them draws the colour into the surface unevenly, leaving a patchy “dry” finish — known in the trade as “pulling”. Before any topcoat, apply a mist coat: emulsion diluted 3:1 (paint:water) brushed thinly over the repaired area (or the whole wall, if newly plastered). Allow to dry fully before the finish coats.
Cost Summary
| Item | Approximate Cost |
|---|---|
| Fine surface filler (500 g tub) | £4–£10 |
| Flexible decorator’s filler (600 g cartridge) | £5–£9 |
| PVA adhesive (1 litre) | £4–£8 |
| Jointing tape (50 m roll) | £4–£7 |
| Mesh tape for plasterboard joints | £3–£6 |
| Professional decorator (full room prep + paint) | £150–£350/day |
Common Mistakes
Filling over dust or loose plaster. The filler simply pulls away when it dries. Always clean and prime the crack first.
Using the wrong filler. A rigid powder filler on a movement crack will re-crack within weeks. Match the filler flexibility to the type of crack.
Not allowing adequate drying time. Painting over filler that is still damp traps moisture, causes bubbling, and creates damp patches that show through the topcoat.
Skipping the mist coat. The single most common reason for a patchy finish — always mist repaired areas and fresh plaster before topcoating.
Overfilling. A mound of filler is harder to sand flat than a slightly shallow fill — two coats are easier than one thick application.
For walls with extensive cracks or widespread surface defects, applying a full coat of finishing plaster or a skimcoat over lining paper may produce a better result than filling individual cracks. A professional plasterer charges £150–£250 per day and can skim an average room in one day, delivering a finish difficult to match by sanding and filling alone.
