Plaster Repair Before Painting: A London Homeowner's Guide

Every painter knows that the quality of the finished surface is determined by what lies beneath. In London's period properties, that means plaster, and the condition of the plaster is often the single most important factor in how good (or how bad) a paint job will look. Cracked plaster, damp-damaged plaster, blown plaster that has separated from its substrate: these are the challenges that must be addressed before a brush touches the wall, or the new paint will fail just as the old paint did.

As professional painters and decorators with extensive experience across London's finest residential areas, from Belgravia and Chelsea to Kensington and Knightsbridge, we encounter plaster problems on virtually every project. Our plaster repair service ensures that every surface is brought to a sound, smooth condition before any decorating begins. This guide explains what you need to know.

Understanding London's Plaster Types

Lime Plaster

Properties built before approximately 1920 typically have lime plaster. This traditional material is a mix of lime (either lime putty or hydraulic lime), sand, and often animal hair as reinforcement. Lime plaster was applied in multiple coats over lath (thin strips of riven or sawn timber nailed to the studs or joists) on timber-framed walls and ceilings, or directly onto masonry walls.

Lime plaster has several characteristics that are relevant to painters:

Flexibility. Lime plaster is softer and more flexible than modern gypsum plaster. This is a virtue in older buildings, which move and settle over time. Lime plaster can accommodate slight movement without cracking catastrophically.

Breathability. Lime plaster is vapour-permeable, meaning moisture can pass through it. In older buildings with solid walls (no cavity), this breathability is essential. If moisture cannot escape through the plaster, it becomes trapped in the wall, leading to damp, salt efflorescence, and plaster failure.

Alkalinity. Fresh lime plaster is highly alkaline. If painted before it has fully cured and carbonated, the alkalinity can attack oil-based paints and some water-based paints, causing discolouration, peeling, and a chalky residue.

Age and condition. In a Georgian townhouse in Belgravia or a Victorian villa in Kensington, the lime plaster may be over a hundred and fifty years old. Despite its age, lime plaster in good condition can continue to function perfectly well. The key is moisture: lime plaster that has been kept dry will last indefinitely, while lime plaster that has been subjected to persistent damp will deteriorate.

Gypsum Plaster

Properties built or renovated from the 1930s onwards typically have gypsum plaster (Carlite, Thistle, or similar brands). This sets by chemical reaction rather than by drying, producing a hard, smooth surface that is excellent for painting.

Gypsum plaster is less forgiving of building movement than lime, and it is not breathable. In older buildings, applying gypsum plaster over a solid brick wall can trap moisture and cause problems. However, in appropriate applications (plasterboard, modern blockwork, dry internal walls), gypsum plaster provides an ideal surface for decoration.

Mixed Plaster

Many London properties have been patched and repaired over the decades, resulting in walls that contain both lime and gypsum plaster. This mixed substrate requires careful treatment, as the two materials have different properties and react differently to moisture, fillers, and paint.

Common Plaster Problems

Cracks

Cracks in plaster are the most common problem we encounter. They fall into several categories:

Hairline cracks in lime plaster are normal and generally insignificant. They result from the natural drying and slight movement of the building. These cracks can be decorated over using a flexible filler and do not indicate a structural problem.

Wider cracks (more than 2mm) may indicate structural movement, settlement, or thermal expansion. Before filling these cracks, we assess whether the movement is historic (and therefore complete) or ongoing. Ongoing movement requires investigation by a structural engineer before any decorating work proceeds.

Pattern cracking (also called map cracking or crazing) appears as a network of fine cracks across the surface. This typically results from the plaster drying too quickly during application, or from a render that is too strong for the substrate. Pattern cracking in gypsum plaster may also indicate that the plaster has got wet and partially dissolved.

Cracks following lath lines indicate that the plaster has lost its key to the lath beneath. This is the beginning of the process that leads to blown plaster (see below).

Blown Plaster

Blown plaster is plaster that has separated from its substrate but has not yet fallen off. On lath-and-plaster walls and ceilings, the lime plaster is held in place by "nibs," small blobs of plaster that squeezed through the gaps between the laths when the plaster was originally applied and hardened behind them. Over time, these nibs can break, vibration can loosen the bond, and moisture can weaken the lime.

You can identify blown plaster by:

Tapping the wall. Sound plaster produces a solid thud; blown plaster sounds hollow and drumlike.
Pressing gently. Blown plaster feels springy and may visibly flex under pressure.
Visual signs. Bulging, cracking along lath lines, and loose areas around the edges of sound plaster.

Blown plaster cannot simply be painted over. It must either be reattached (using specialised plaster washers and adhesive injection) or removed and replaced. The choice depends on the extent of the problem, the significance of the building, and the condition of the lath behind.

Damp Damage

Damp is the great enemy of plaster in London properties. Common sources include:

Rising damp from the ground, particularly in basement and lower-ground-floor rooms of townhouses in Belgravia, Pimlico, and Chelsea. Rising damp brings soluble salts from the ground into the plaster, where they crystallise as the moisture evaporates, causing the plaster to crumble and the paint to flake.

Penetrating damp from defective roof coverings, flashings, guttering, and downpipes. A single leaking gutter can cause damp damage to plaster over several floors as water tracks down within the wall.

Condensation in bathrooms, kitchens, and poorly ventilated rooms. Condensation affects the surface of the plaster, promoting mould growth and paint failure. Extended condensation can soften gypsum plaster.

Plumbing leaks. Even a slow leak from a concealed pipe can saturate plaster over months or years, causing extensive damage before the source is discovered.

In all cases, the source of the damp must be identified and rectified before any plaster repair or painting is undertaken. Repairing plaster and applying new paint over a damp wall is a waste of time and money; the problem will recur.

Salt Efflorescence

Salts crystallising on the surface of plaster appear as white, fluffy or crusty deposits. This is a common problem in London's period properties, particularly at lower levels. The salts are drawn from the masonry, mortar, or soil by moisture, and they crystallise at the surface as the water evaporates.

Salt efflorescence damages paint by pushing it off the surface from behind. Before painting, salts must be brushed off (dry brushing only; do not wash, as this drives the salts back into the wall), and the wall must be allowed to dry out fully. In severe cases, the contaminated plaster may need to be removed and replaced with a salt-resistant specification.

Plaster Repair Methods

Crack Repair

For hairline cracks in stable plaster, the standard approach is:

Open the crack slightly using a filling knife or scraper to create a key for the filler
Brush out any loose material and dust
Apply a flexible filler (such as Toupret Fibacryl or a similar acrylic filler) using a filling knife, pressing the filler into the crack
Allow to dry fully (this may take twenty-four hours for deeper fills)
Sand smooth with fine abrasive paper (120 to 180 grit)

For wider cracks that have been assessed as stable, the same process applies, but the crack may need to be filled in stages to avoid shrinkage. For cracks over 5mm wide, we embed a fibreglass mesh tape in the first coat of filler to prevent the crack reopening.

Patching and Re-plastering

Where plaster has been removed or has fallen away, the area must be patched to match the surrounding surface.

On lime plaster walls, patches should ideally be carried out in lime plaster, particularly in listed buildings and properties where breathability is important. Lime plaster patching is a specialist skill: the mix proportions, the number of coats, and the curing process all differ from gypsum work. We use lime plaster for our heritage painting projects in conservation areas and listed buildings.

On gypsum plaster walls, patches are straightforward using multi-finish or similar gypsum plaster. The key to a good patch is feathering the edges so that the junction between old and new plaster is invisible.

On mixed walls, the choice of patching material depends on the specific situation. We assess each case individually and choose the material that will provide the best long-term result.

Skim Coating

Where the existing plaster is sound but the surface is rough, uneven, or badly scarred by years of filling and sanding, a skim coat may be the best solution. This involves applying a thin (2 to 3mm) layer of finishing plaster over the entire surface, producing a perfectly smooth, uniform finish.

Skim coating is particularly valuable when preparing walls for luxury wallpaper or decorative finishes, where any surface imperfection will be visible.

Ceiling Repairs

Ceiling plaster repair follows the same principles as wall repair, but with added complications. Gravity works against the plasterer on ceilings, and the consequences of failure are more dramatic (and more dangerous) than on walls.

For lath-and-plaster ceilings, blown areas must be carefully assessed. If the ceiling is extensively blown, it may be safer and more practical to take it down and replace it with plasterboard and skim, rather than attempting piecemeal repairs. However, in listed buildings or properties with decorative plasterwork, retention of the original ceiling is strongly preferred, and specialist repair techniques (including plaster washer installation from above where accessible) can be used.

Our ceiling painting team is experienced in assessing ceiling plaster conditions and carrying out necessary repairs before decoration.

Preparing Repaired Plaster for Painting

Drying Time

New plaster must be thoroughly dry before painting. The standard rule of thumb is one day per millimetre of plaster thickness, plus one week for good measure. A 12mm skim coat should be left for a minimum of three weeks in warm, well-ventilated conditions. In winter, or in poorly ventilated rooms, this can extend to six weeks or more.

Do not be tempted to rush this stage. Painting over damp plaster traps moisture, causes adhesion failure, and can lead to mould growth behind the paint film.

Mist Coat

New plaster (whether lime or gypsum) is highly absorbent and must be sealed before the finish coats are applied. The traditional method is a mist coat: matt emulsion thinned with water (typically 70:30 or 60:40 paint to water). The mist coat soaks into the plaster, binding the surface particles and providing a key for subsequent coats.

For lime plaster, use a breathable emulsion for the mist coat. Standard vinyl emulsions can form a non-breathable film that defeats the purpose of using lime plaster. Little Greene Intelligent Emulsion and Earthborn Claypaint are both suitable for use on lime.

Priming Over Stain and Salt Damage

Where plaster has been affected by damp staining, nicotine, tannin bleed, or salt contamination, a specialist primer is needed before decorating:

Zinsser BIN (shellac-based) for stain blocking on isolated areas
Zinsser Gardz for sealing powdery or damaged plaster surfaces
Dryzone or similar anti-salt primer for walls affected by salt efflorescence

These primers create a barrier that prevents stains and salts from bleeding through the finish coats.

When to Call in a Specialist

Plaster repair is within the competence of any good painter and decorator for minor cracks and surface defects. However, certain situations require specialist plaster skills:

Decorative plasterwork repair (cornices, ceiling roses, mouldings) where damaged elements must be reproduced to match the original
Lime plaster application on listed buildings or in conservation areas, where modern materials are inappropriate
Extensive ceiling repairs where structural considerations are involved
Damp investigation and treatment, which may require input from a surveyor or damp specialist

Our team includes experienced plasterers who work alongside our interior painting decorators, ensuring that plaster repairs and painting are coordinated seamlessly. We do not treat plaster repair as someone else's problem; it is an integral part of our decorating service.

Contact Us About Plaster Repair and Painting

If your London property needs plaster repairs before painting, whether minor crack filling or major restoration of lime plaster in a listed building, we can help. Our integrated approach means one team handles both the repair and the decoration, ensuring consistent quality from substrate to finish coat.

Contact us to arrange a survey and assessment of your plaster condition. We serve properties across Belgravia, Chelsea, Kensington, Knightsbridge, Pimlico, and the wider London area.

Plaster Repair Before Painting: A London Homeowner's Guide