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Multi Finish Plaster
What multi finish plaster is, which backgrounds it suits, how it differs from board finish, coverage, mixing, application and common problems.
What plastering is and what it does for a wall
Plastering gives an interior wall a continuous surface that can be painted or otherwise decorated. The term covers both the plaster applied to a background and the work of forming its finished face. Internal plaster, external render and plasterboard are related wall finishes, but differ in material, position and construction. Knowing those distinctions helps you choose a finish that suits the building and the surface beneath it.
Plaster as the finished surface of an interior wall
Plastering is the application of one or more thin coats of wet plaster to a wall or ceiling to create a flat, continuous, paintable surface. A practical reference here is NV Casino. The plaster may be applied directly to masonry or over plasterboard; the substrate and chosen system determine the materials and total thickness. A finished coat gives the room a consistent face for paint or wallpaper and covers the joints between plasterboard sheets when the board is skimmed.
In trade language, a backing coat is the layer beneath the finish, while a finish coat or skim is the fine surface layer. On masonry, an undercoat may also be called a floating coat; the plasterer rules it level before applying the finish. A skim coat is commonly about 2–3 mm thick. An undercoat is generally around 8–11 mm, depending on whether it is on a ceiling or wall and on the system used. These are typical ranges, not a specification for every product or background.
The surface can be finished with a steel trowel to provide a smooth plaster finish, or given a texture where the chosen decoration calls for it. A sound, even finish makes paint application more uniform and gives skirting, architraves and other room details a clean junction with the wall. Plaster also forms part of a wall assembly, so its contribution to fire or acoustic performance depends on the complete system rather than the skim coat alone.
For your project, identify whether the intended finish is a thin skim over an existing base or a complete plaster build-up; that distinction affects the materials and finished thickness.
Plaster, render and plasterboard compared
Internal plaster is a wet-applied finish or build-up on an inside wall, whereas render is a coating for external masonry and plasterboard is a factory-made sheet lining. Their thicknesses and performance depend on the full system, but the typical figures below help distinguish them.
| Material | Composition and location | Typical thickness | Advantages | Limits and common uses |
|---|---|---|---|---|
| Internal plaster | Gypsum- or lime-based wet mix, applied indoors to masonry, lath or board. | Finish skim about 2–3 mm; an undercoat is commonly about 8–11 mm. | Forms a continuous face and can follow the contours of a properly levelled background. | Needs a suitable substrate and skilled application; used on internal walls and ceilings, either as a build-up or a finish coat. |
| External render | Typically cement-based or lime-based mortar applied to the exterior of masonry. | Often about 15–20 mm overall, varying with the specified render system. | Provides a weather-exposed protective coating and can be finished with different textures. | Must suit the wall and exposure; it is used on external facades, not as an indoor gypsum skim. Gypsum plaster is not used outdoors. |
| Plasterboard | A gypsum core faced with paper, fixed to framing or masonry as an internal lining. | Standard UK sheets are commonly 12.5 mm thick and 2400 × 1200 mm; a skim, if specified, adds about 2–3 mm. | Provides a fast, flat lining with predictable sheet dimensions and can be fixed to a framed partition. | Joints and fixings need a suitable finish; it is used for partitions, ceilings and wall linings rather than as a wet-applied coating. |
The key distinction is that plaster and render are applied as wet coatings, while plasterboard is a sheet material. Choose among these materials for plastering by considering whether the surface is inside or outside, the type of background and whether the wall needs a coating or a board lining.
Gypsum plaster versus lime plaster
Gypsum plaster is made from calcined gypsum, calcium sulphate hemihydrate, and sets by a chemical reaction with water. Lime plaster sets slowly by carbonation, absorbing carbon dioxide from the air; hydraulic lime can also gain strength through hydraulic reactions. The different setting behaviour affects working time, background compatibility and where each material is appropriate.
| Characteristic | Gypsum plaster | Lime plaster |
|---|---|---|
| Composition and setting | Calcined gypsum, or calcium sulphate hemihydrate, reacts with water and forms a chemical set. | Lime putty or lime-based mortar gains strength gradually through carbonation; hydraulic lime also sets through hydraulic reactions. |
| Typical setting or curing | Many finishing plasters set in roughly 1.5–2 hours, depending on product and site conditions; drying before decoration takes longer. | Carbonation proceeds over weeks or longer, with the rate affected by coat thickness, moisture and air movement. |
| Suitable backgrounds | Commonly used on suitable internal masonry and plasterboard in dry conditions. | Often chosen for porous traditional masonry and lath where a compatible, more vapour-permeable finish is needed. |
| Typical choice | Useful for many modern interiors where a relatively quick-setting, smooth finish is required. | Suited to compatible older buildings and traditional construction where the wall needs a lime-based system. |
| Important limitation | Not suitable for external exposure; gypsum plaster is not used outdoors. | Slower carbonation and drying mean the work needs a suitable curing period and compatible detailing. |
Gypsum is often selected for standard dry interior work, while lime is chosen for compatible traditional backgrounds and building fabric. Match the plaster to the wall rather than choosing by setting speed alone.
The stages of a plastering job
A plastering job depends on matching the treatment to the wall beneath the finish, rather than applying the same mix and thickness everywhere. The background determines how quickly water leaves fresh plaster and whether an undercoat can grip securely. A measured undercoat provides a level base; a thinner finish coat creates the surface ready for decoration. Setting and drying are separate processes, so a wall that feels firm may still need time before paint is applied. The stages below explain the checks, coat sizes and drying cues that help produce a smooth plaster finish.
Preparing the background and controlling suction
Start by identifying the background and checking that it is stable, clean and suitable for the intended plaster system. Brick and blockwork can draw water rapidly, plasterboard has a paper face with comparatively even suction, and old plaster may vary across a single wall.
- On brickwork, check for loose mortar, dust and absorbent areas; brush away debris and repair open joints before applying a compatible undercoat.
- On blockwork, note whether blocks are lightweight or dense: lightweight units can absorb water quickly, while dense surfaces may need a different bonding approach.
- On plasterboard, make sure boards are firmly fixed, joints are supported and the face paper is intact; use a board-compatible finish system.
- On old plaster, tap and inspect for hollow, powdery or detached areas; remove unsound material and assess any sound painted surface for adhesion.
- For high-suction masonry, apply a suitable diluted bonding agent or suction-control primer evenly, following its product label rather than assuming one dilution suits every wall.
The aim is even suction, not a sealed, shiny surface. Where PVA is specified, dilute it to the manufacturer’s instructions and apply the plaster at the stated stage—commonly while the coat remains tacky, not after it has dried to a slick film. For a low-suction or smooth background, use a compatible bonding coat or mechanical key where the plaster system calls for one; a high-suction surface may instead need suction control and a suitable undercoat. Choose the undercoat by substrate, exposure and manufacturer compatibility, then confirm the preparation method in the product data sheet before mixing.
Undercoat and finish coat
The undercoat builds the plane of the wall and gives the finish coat a consistent base. For many internal gypsum systems, a typical undercoat is about 8–11 mm thick; the finish coat is much thinner, usually around 2–3 mm. Product instructions take precedence because substrate and system affect the recommended build.
- Choose an undercoat suited to the background, such as a masonry undercoat for compatible brick or block, or a bonding undercoat for a suitable smooth, low-suction surface.
- Mix with clean water in the proportions stated by the manufacturer; add powder to water and mix to a lump-free consistency using clean equipment.
- Apply the undercoat to the required build, using dots and screeds or a straightedge to control thickness and bring the wall to a flat plane.
- Where the system requires a key, rule the coat level and scratch it lightly while workable; avoid deep grooves that cut through the coat.
- Apply the finish coat over the prepared undercoat at about 2–3 mm, keeping the layer even and working the surface as it firms.
Keep thickness consistent rather than trying to correct a deep hollow with an excessive single pass; follow the chosen plaster’s stated limits and use the appropriate backing coat where more build is needed. A typical finish sequence uses a first thin application followed by a second pass to fill and level, then trowelling as the material firms. British Gypsum gives Thistle MultiFinish coverage of approximately 10 m² per 25 kg bag at 2 mm thickness (British Gypsum, Thistle MultiFinish product data sheet, accessed 2025). Treat this as a planning estimate: actual coverage changes with the wall’s flatness, waste and applied thickness.
Drying, curing and decorating
Setting and drying describe different changes. Gypsum plaster sets through a chemical reaction that makes it firm enough to work and finish; drying follows as excess moisture leaves the wall. A plastered surface can therefore feel hard while retaining moisture, and decoration should wait for it to dry rather than rely on touch alone. On pink gypsum plaster, the usual visual cue is a change from dark pink areas to a more even pale pink. Colour can vary between products and backgrounds, so check the plaster manufacturer’s instructions as well as the appearance. Thin coats in a warm, ventilated room may dry within days, while thicker areas, cool conditions or high humidity can extend drying to several weeks. Dulux advises that new plaster can take up to four weeks to dry fully before painting (Dulux, new-plaster painting guidance, accessed 2025). Once the surface is uniformly pale and dry, follow the paint manufacturer’s directions for new plaster. Some systems specify a mist coat or a dedicated plaster primer; use the stated product and dilution rather than applying ordinary paint at full strength by default. Avoid trapping moisture beneath a finish coat, since premature painting can impair adhesion or leave an uneven appearance. If any area remains visibly darker, allow more drying time and reassess before decorating.
Common wall problems that plaster solves
Wall defects range from shallow ridges to plaster that has lost its bond with brick, block or lath. The visible surface is only one clue: tapping, checking edges and looking for recurring cracks can reveal whether the problem extends into the background. A local patch, a re-skim or complete replastering suits different levels of damage. The condition of the existing plaster, the surface texture and any movement in the building determine which repair is likely to last.
Uneven or damaged walls
A re-skim is a thin coat of finishing plaster applied over sound existing plaster; it evens out shallow ridges and worn areas without removing the underlying layer. Replastering means taking the old plaster off back to its background, such as brick, block or lath, before forming a new surface. A skim cannot make loose plaster secure or reliably hide deep hollows, so the first decision is whether the existing material remains firmly attached.
Check the wall for hollow sounds when tapped, bulges, flaking, damp staining and edges that crumble when lightly pressed. A few small blemishes in otherwise solid plaster may need only local filling or a re-skim. If broad areas sound hollow or move under gentle pressure, the old plaster has probably lost its bond; covering it can leave the new coat liable to crack or detach. Extensive failure points towards replastering rather than a cosmetic layer.
The background also matters. Plaster over masonry may fail because of a weak or contaminated surface, while old lath walls can have loose or broken laths behind the finish. Where the background is sound and the irregularity is shallow, a re-skim is usually the less disruptive repair. Deep changes in level, exposed masonry or damaged laths call for removal and a repair suited to the actual background.
Look for signs that the wall itself is moving, including cracks that keep reopening or separation around openings. Building movement can make either a patch or a fresh plaster coat fail if its cause remains active. The choice therefore depends on four checks: the condition of the old plaster, the integrity of the background, the depth of unevenness and whether movement is present. Stable, shallow defects favour a skim; widespread detachment or deep damage favours replastering.
Old textured and painted surfaces
Textured coatings can be removed or covered, but the right approach depends on their composition, bond and relief. A firmly attached, shallow texture may be overcoated after the surface has been assessed and prepared; pronounced peaks can show through a thin skim or make it harder to achieve an even finish. If the texture is loose, flaking or built up in several layers, removal may be needed rather than adding weight over a weak surface.
Some textured decorative coatings applied before the UK asbestos ban in 1999 may contain chrysotile, also known as white asbestos. The Health and Safety Executive (HSE) identifies textured coatings as materials that can contain asbestos; appearance alone cannot confirm whether a particular coating contains it. Testing is the only reliable check, using a sample analysed by a UKAS-accredited laboratory. Do not sand, scrape or drill an untested pre-1999 coating, because disturbing asbestos-containing material can release fibres.
If asbestos is confirmed and the coating is intact, leaving it undisturbed and sealing or covering it can be an option, provided the proposed finish will not damage it. Where removal is necessary, the method and controls depend on the material’s condition and the work involved. Follow HSE asbestos guidance and use a suitably trained asbestos professional to assess the work; do not treat ordinary removal methods as suitable for a coating that may contain asbestos.
For coatings confirmed to be asbestos-free, removal can expose the original wall for repair, while overcoating preserves a sound base and avoids unnecessary stripping. Painted walls need a separate bond check: glossy or peeling paint can prevent a new plaster layer from adhering, and loose paint should not be hidden beneath a skim. Small, firm areas may be prepared locally, but widespread poor adhesion or heavy texture can make removal back to a stable surface the more dependable route.
Choose between removal and overcoating only after checking what the coating is, how securely it is attached and how much relief it has. A thin, sound, asbestos-free texture may suit overcoating; loose layers need to be dealt with first, and suspected asbestos requires testing before disturbance. These checks help avoid trapping an unstable finish beneath new plaster.
Cracks and patches
Fine cracks can follow drying shrinkage, changes in temperature or minor movement, while wider or recurring cracks may indicate movement in the wall or its background. Cracks around doors and windows, diagonal lines from an opening, and a gap that is visibly widening deserve closer inspection than a short hairline mark in an otherwise sound surface. Note whether the crack returns after a previous repair and whether nearby masonry or openings show corresponding damage.
A stable, isolated blemish can usually be cut back to firm edges, filled with a suitable repair material and blended into the surrounding plaster. Where a crack is active or passes through masonry, filling the visible line alone is unlikely to last. Crack stitching uses stainless-steel helical bars set across a masonry crack to tie the affected sections together; it is a structural repair, not simply a plastering treatment, and the cause and extent of movement should be assessed first.
Once the background is stable, a local patch suits damage confined to a small area. A re-skim can cover widespread shallow cracking when the underlying plaster remains sound, but it will not stabilise a moving wall. Replastering is more appropriate when cracks accompany extensive hollow, detached or crumbling plaster, or when removal is needed to reach and repair a damaged background.
Repair choice follows the extent and cause: patch a limited defect, stitch a confirmed masonry crack where appropriate, or re-skim sound plaster with widespread surface marks. If cracks keep widening or reappear after filling, investigate building movement before applying a finish; full replastering is needed when the old plaster has broadly failed, not simply because one fine crack is visible.
Plastering questions answered
Clear answers to common questions can help you choose a suitable plaster finish and plan the work around the wall and its intended decoration.
How long does plaster take to dry?
Plaster may set firm within about 1–2 hours, but setting is not the same as drying; allow up to four weeks for a typical new plaster finish to dry fully, as advised in British Gypsum guidance (2024). The actual time depends on coat thickness, ventilation, room temperature and moisture in the background. Look for a consistently pale pink or light colour across the wall; darker patches usually indicate remaining moisture. Keep the room gently ventilated and avoid forceful heating, which can dry the surface unevenly.
Can you plaster over old paint?
Plaster can sometimes go over paint that is firmly bonded, clean and free from grease, but a weak or flaking coating must be removed first. Check adhesion by scraping several areas and pressing firmly with a suitable tape; paint that lifts, blisters or powders is not a sound base. Remove loose material, wash off grime, and abrade glossy paint to create a key before applying a compatible bonding primer. Test the primer and plaster on a small patch and check that the coat bonds and dries without lifting before covering the whole wall.
Is lime plaster better than gypsum plaster?
Neither is universally better: lime suits solid-walled traditional buildings, while gypsum suits modern masonry and plasterboard. Lime plaster is more vapour-permeable and can accommodate moisture movement in older walls, helping them dry without trapping moisture behind a dense finish. Gypsum sets relatively quickly and produces a smooth, hard surface, but is less suitable where a traditional solid wall needs to release moisture. Match the plaster to the construction and existing materials rather than choosing by finish alone.
Do plasterboard walls need plastering?
No; plasterboard can be left taped and jointed for painting, or skimmed with a 2–3 mm layer of finish plaster, a typical thickness described in British Gypsum guidance (2024). A taped-and-jointed finish suits boards installed accurately when the joints and fixings can be filled and sanded to the required standard. Skimming is useful when you want a continuous, uniform surface across the board faces and joints. Choose the finish to suit the board condition and the appearance required before decorating.