Technical & Product Guidance
Filling Cracks and Plaster Repair Before Decorating
Every painter knows that the quality of the finish depends on the quality of the surface beneath it. In London's older properties — Georgian townhouses, Victorian terraces, Edwardian mansion blocks — the surfaces are rarely perfect. Cracks, blown plaster, previous repairs of varying quality, and decades of paint build-up are all common. Dealing with these properly before the first coat goes on is the work that makes everything else look right.
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Filling Cracks and Plaster Repair Before Decorating
Assessing the condition: what to look for
Before any filling or repair begins, a systematic assessment of every surface is essential. This means checking walls and ceilings by both visual inspection and touch. Tap the surface with knuckles — solid plaster gives a dull thud, blown or detached plaster sounds hollow. Run a hand over the surface to feel for bulges, soft spots, and changes in texture that may not be visible under existing paint. Cracks fall into several categories, and the type determines the repair method. Fine hairline cracks in the paint surface are often cosmetic and can be addressed with a flexible filler and redecoration. Wider cracks that follow a straight line — along the junction of a wall and ceiling, around door frames, or along the line of a partition — typically indicate structural movement or differential expansion. Irregular, branching cracks through the plaster itself may indicate a more significant substrate problem. Damp staining, efflorescence — the white crystalline deposits that appear on masonry — and soft, powdery plaster surfaces all need to be identified before work begins. These are symptoms of underlying moisture issues, and simply filling and painting over them is a waste of time and money. The source of moisture must be addressed first. A professional decorator will flag these issues to the client or managing agent rather than painting over problems that will inevitably return.
Filling cracks: methods and materials
Hairline cracks in otherwise sound plaster are best filled with a fine surface filler applied with a flexible knife, sanded smooth when dry, and spot-primed before painting. Products like Toupret Fine Surface Filler or Gyproc Easi-Fill are suitable for this work. The filler should be applied slightly proud of the surface and sanded back flush, leaving an invisible repair that will not telegraph through the finished paint. Wider cracks — anything over about two millimetres — need more preparation. The crack should be opened out with a scraper or filling knife to create a V-shaped channel, loose material removed, and the edges dampened before filling. For cracks caused by movement, a flexible acrylic filler is preferable to a rigid plaster filler, as it will accommodate future minor movement without cracking again. Caulk is sometimes used for corner junctions, but it should not be painted over with matt emulsion as it shows through as a shiny line. Recurring cracks — those that have been filled before and have reopened — indicate ongoing movement. Filling them again with the same approach will produce the same result. Options for persistent cracks include using a reinforcing tape such as self-adhesive fibreglass mesh embedded in filler, lining the area with a heavy lining paper before painting, or in severe cases, cutting out the cracked plaster and patching with fresh material. The right approach depends on the cause and severity, and an experienced decorator will advise on the most appropriate solution.
Dealing with blown and damaged plaster
Blown plaster — where the plaster has detached from the wall behind it but has not yet fallen away — is extremely common in London's older buildings. It creates a hollow drum-like sound when tapped and may show as a bulge or an area of cracking on the surface. Small areas of blown plaster can sometimes be stabilised by injecting PVA adhesive behind the plaster and pressing it back, but this is only a temporary solution for minor areas. The proper repair for blown plaster is to cut out the affected area back to sound material, clean the exposed substrate, and patch with fresh plaster. In period properties with lime plaster, the patch should ideally be made with a lime-based product to maintain compatibility with the original material. Using modern gypsum plaster to patch lime plaster can create issues with differential expansion and moisture behaviour. A good plasterer — not the same as a good decorator — should carry out larger repairs. Skim coating is the appropriate treatment where the plaster surface is generally sound but has too many imperfections — filled cracks, old wallpaper damage, textured finishes, or general roughness — to produce a good painted finish. A thin skim of finishing plaster over the existing surface creates a new, smooth base for decoration. This adds a day or two to the programme for drying and preparation, but the improvement in the finished result is significant. On ceilings in particular, a fresh skim coat is often the only way to achieve a truly flat, defect-free surface.
Surface preparation after repair
Once all filling and plastering work is complete and has had adequate drying time, the surfaces need final preparation before painting. All filled areas should be sanded smooth and flush with the surrounding surface. Use a fine-grit sandpaper — one-hundred-and-twenty to one-hundred-and-fifty grit — and check the smoothness by running a hand over the surface and by viewing it at a low angle, which reveals any bumps or depressions that a direct view might miss. New plaster and large filled areas need a mist coat — a diluted first coat of emulsion, typically mixed fifty-fifty with clean water — which seals the porous surface and provides a key for subsequent coats. Without a mist coat, the full-strength emulsion dries too quickly on the porous surface, creating a patchy, poorly adhered finish. Bare plaster that has been mist-coated and allowed to dry overnight will accept the finishing coats evenly and predictably. Previously painted surfaces in good condition need washing down to remove grease, dust, and atmospheric deposits. Sugar soap solution applied with a sponge, rinsed with clean water, and allowed to dry is the standard approach. Any glossy surfaces that are to receive matt emulsion should be lightly sanded to provide a mechanical key. These preparation steps are unglamorous but essential — skipping them in the interest of speed leads to finishes that look acceptable on day one but deteriorate within months.
When to call in a specialist
Not all plaster repair is decorator's work. Where the damage involves large areas of failed plaster, where the substrate behind the plaster is compromised — crumbling brick, failed lime mortar, damp masonry — or where the property has heritage features that need conserving, a specialist plasterer should be involved. A decorator who is honest about the limits of their expertise is worth more than one who attempts repairs beyond their skill level. In listed buildings and conservation areas across Westminster, plaster repair may need to follow specific methods and use traditional materials. Lime plaster repairs, cornice restoration, and the reinstatement of period mouldings require craft skills that are distinct from standard decorating. The decorator and the plasterer should work in sequence — repairs completed and dried, then decoration follows — to produce the best result. Damp-related plaster failure — where the plaster is soft, powdery, or salt-contaminated — requires investigation and treatment of the moisture source before any replastering or decoration. This may involve a damp specialist, a surveyor, or remedial work to external drainage, pointing, or tanking. The decorator should not be expected to diagnose or resolve the underlying damp problem, but they should recognise it and raise it before wasting time and materials decorating over an active issue.
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Frequently Asked Questions
Common questions about this topic.
Tap the surface with your knuckles. Sound plaster gives a dull thud; blown plaster sounds hollow. If the area of hollow plaster is small — less than about thirty centimetres across — it may be possible to stabilise it. Larger areas, or plaster that is soft, crumbling, or visibly detached, need cutting out and replastering.
Paint will bridge very fine hairline cracks temporarily, but they will show through again quickly. Wider cracks must be filled before painting, as paint cannot bridge them and the result will look poor from day one. Proper filling adds minimal time to the project and makes a significant difference to the finished result.
Standard gypsum plaster should dry for at least four weeks before painting, though this varies depending on thickness, ventilation, and room temperature. In winter, drying takes longer. The plaster should have changed from dark pink or brown to a uniform pale colour and should feel dry to the touch. A moisture meter reading below five per cent confirms it is ready.
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