How to Fill Cracks Before Painting

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 and cracks that may signal movement: the Building Research Establishment treats cracks up to about 5 mm wide as aesthetic damage that needs only redecoration, while wider, diagonal or growing cracks need professional assessment before any filling work begins.¹ 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

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. Clear any dust left in deep cracks with a dry paintbrush.

Step 3: Prime with PVA

Many ready-mixed fillers need no primer, but on dusty or very absorbent old plaster a coat of diluted PVA seals the suction, stops the filler drying too fast and improves adhesion.² Dilute it well (Everbuild’s data sheet for its PVA gives 4 parts water to 1 part PVA for sealing porous plaster) and brush it into the crack and onto the surrounding surface.³ 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 Multi Purpose Smooth Finish Filler, £4–£10 a 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 it slightly proud of the surface so it can be sanded flush.

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 it to dry fully (check the drying time on the product), 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): Build the repair up in layers rather than one thick fill. Ronseal, for example, tells users to fill deep holes in layers and let each layer dry before adding the next.⁴ For larger voids, use bonding plaster or multi-finish plaster instead. 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, sand the repair flat using 120-grit paper on a sanding block. Ready-mixed fillers dry quickly: Ronseal gives a drying time of one to two hours for its Multi Purpose Smooth Finish Filler, while deep repairs, powder fillers and cold or damp rooms take longer.⁴ 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 thinned with water (Dulux suggests ten parts paint to one part water; follow the instructions on your own tin) brushed thinly over the repaired area (or the whole wall, if newly plastered).⁵ Allow to dry fully before the finish coats.

Cost Summary

A 500 g tub of Polycell Trade fine surface filler was £9.29 including VAT at Toolstation in September 2026.²

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 typically charges £200–£300 per day and can often skim an average room in a day or two, delivering a finish difficult to match by sanding and filling alone.