Repairing and Painting Coving and Cornices: A Practical Guide
How to patch plaster cornice versus replace with fibrous plaster, which fillers to use, and how to achieve a clean paint edge at the cornice-to-wall junction.
Why cornice repair matters before painting
Redecorating a room with damaged or poorly prepared coving and cornices is a false economy. Cracks, loose sections, missing enrichments, and a ragged paint edge at the junction between cornice and wall will all draw the eye regardless of how perfect the wall finish is. Period properties in London — Belgravia townhouses, Chelsea mansion flats, Kensington Victorian terrace houses — derive much of their character from well-maintained plasterwork. Getting the repair right before a brush touches the decorative coats is the foundation of a result worth having.
Assessing the damage: what do you have?
Before deciding on a repair approach, understand what type of cornice is in place and how extensive the damage is.
Tap along the cornice with your knuckle. A hollow sound indicates the plaster has detached from the key — the background plaster or the lathing behind it. A solid sound indicates good adhesion. A hollow section that cannot be re-bonded by re-screwing or re-injecting with a plaster bonding adhesive is a candidate for replacement rather than repair.
Examine the profile. Traditional run-in-place cornices have a continuous profile worked with a template while the plaster was wet. Fibrous plaster cornices (factory-cast) have a sharper, more consistent profile. Knowing which you have helps determine the repair approach: a run cornice can be patched with lime plaster and re-run; a fibrous cornice is best patched with casting plaster or replaced with a matching section.
Check for the overall condition of the plasterwork above. If the ceiling plaster is old and in poor condition, extensive cornice repair before the ceiling is done may be wasted effort — plan the sequence carefully.
Patching plaster cornice in place
For small areas of damage — short missing sections, surface spalling, areas where previous repairs have failed — patching in place is appropriate. The tools needed are a profile gauge or paper template of the undamaged cornice section, a gauging trowel, and the appropriate plaster mix.
For lime plaster cornices (the original material in pre-1900 London houses), repair with a mix of fine lime putty and sharp sand (1:1) stiffened with a small amount of gypsum plaster of Paris for set time. Apply in layers no more than 5 to 6mm thick, scoring each layer before applying the next. The final layer should be worked with the profile template — a piece of sheet metal or stiff card cut to the cornice profile — to match the existing shape.
For later gypsum plaster cornices, use Thistle Finishing Plaster or a pre-mixed coving adhesive as the repair medium. Work in layers, keying back between coats, and bring the surface level to match the surrounding area before tooling to the profile.
Replacing sections with fibrous plaster
Where damage is extensive, or where the existing cornice has been so heavily painted over that the profile detail is lost, replacement with a new fibrous plaster section is the better option.
Fibrous plaster cornices are manufactured to standard profiles and are available from specialist suppliers including Stevensons of Norwich, Locker & Riley, and various London plasterers' merchants. Measure the existing profile carefully and order the matching section; for non-standard profiles in listed or conservation properties, bespoke casting from a mould taken from the undamaged section is available.
Fix new fibrous plaster sections with coving adhesive — Gyproc Coving Adhesive or Sika MonoTop as alternatives — applied to the back of the section and to the prepared wall and ceiling surfaces. Butt joints between sections should be filled with the same adhesive and tooled smooth. Scrim tape over joints before plastering adds strength at the junction.
Filler choice for hairline cracks
Hairline cracks at the cornice-to-wall and cornice-to-ceiling junctions are universal in London period properties. They result from differential thermal and moisture movement between the cornice and the background. The wrong filler — a hard-setting rigid product — will simply crack again within a season.
Decorators' caulk (Soudal Poly Max, UniBond Never Crack Caulk, Geocel Trade Mate) is the correct product for moving junction cracks. It remains permanently flexible, accepts paint, and can be smoothed with a wetted finger before curing. Apply with a caulk gun, tool the surface smooth while wet, and allow to cure before painting. Do not paint until the caulk has fully skinned — check the manufacturer's specification, usually 24 to 48 hours.
For surface cracks in the body of the cornice (not at junctions), a fine-surface filler is appropriate: Toupret Finishing Filler, Knauf Uniflott, or Polycell Fine Surface Filler. Apply, allow to dry, sand back to a smooth finish with 120-grit, and prime before painting.
Achieving a clean paint edge at the cornice junction
The line between cornice and wall is one of the hardest painting details to execute cleanly by hand. The angle is not always consistent; the cornice profile projects toward the room; and the shadow gap at the junction magnifies any imperfection.
The most reliable method is to use the cornice profile itself as your cutting guide. Load a 1.5-inch angle sash cutter (Hamilton Perfection or Purdy XL Cutter) with paint on the top third of the bristles only. With the bristle fan pressed into the junction angle, draw the brush steadily along the line. The bristles under compression will follow the angle and deposit paint exactly at the junction.
Where the existing cornice is very close to the wall colour — or painted the same colour — the cut line is less critical. A two-tone scheme (cornice in a different colour from the wall, or white cornice against a coloured wall) requires absolute precision at the junction and is best cut in without masking tape, as tape on an irregular plaster surface rarely gives a cleaner result than a skilled freehand cut.
Primer choice before painting
New plaster repairs should be sealed with a diluted emulsion mist coat (70% emulsion, 30% water) before topcoats. New fibrous plaster sections need the same treatment. Where plaster repairs adjoin previously painted surfaces, Zinsser Bulls Eye 1-2-3 over the repair area prevents a visible matt patch in the topcoat.
Any staining — watermarks, old tar or oil residues, knot staining — should be treated with Zinsser BIN shellac primer before the topcoat to prevent bleed-through.
For a professional assessment of your cornices and coving, contact us or request a free quote. We specialise in period property decoration throughout central London.