Plastering and Decorating in a Renovation: A Practical Guide

Modern domestic plastering uses a sequence of coats, each with a specific function:

**Backing coats (the base layers)**:

• *Hardwall (lightweight backing plaster)*:
• •Most common modern backing coat for masonry walls
• •Mixed to a creamy consistency, applied 10–15mm thick as a single coat on brick or blockwork
• •Sets hard, providing the flat substrate for the skim coat
• •Brands: British Gypsum Thistle Hardwall (the industry standard), Knauf Hardwall
• •Not suitable for: curved surfaces, very uneven substrates, or where more than 20mm buildup is needed

• *Bonding coat (browning plaster)*:
• •Used on less porous substrates — concrete, aerated blockwork (Celcon/Toplite), plasterboard on metal stud, or surfaces where Hardwall wouldn't adhere reliably
• •Lighter than Hardwall, better adhesion on low-suction surfaces
• •Often used as the first coat on aerated blocks, followed by a second Hardwall coat, then skim

• *Carlite browning / Carlite bonding*:
• •Older British Gypsum product names for browning/bonding coats — still in common use among experienced plasterers

• *Sand and cement scratch coat*:
• •Used in wet areas (below tiles) or external walls — sand:cement mix (3:1 or 4:1) provides a waterproof base that is unsuitable for standard gypsum finishes but ideal as a tile bed
• •Also used as the first coat in a three-coat render system on external walls

**Finish coat (skim)**:

• *Thistle Finish / Multifinish (finish skim)*:
• •The final 2–3mm coat applied over backing coats or directly over plasterboard (board finish)
• •Applied in two thin passes and trowelled to a smooth, tight, featureless surface
• •Drying time: typically 24–48 hours to touch; 4–6 weeks to 'fully dry out' for painting without risk of alkaline burn-through
• •New plaster that is painted too soon will show colour banding, blotching, and paint failure

• **Plasterboard and dot-and-dab**:
• •Plasterboard (Gyproc/Knauf 12.5mm) fixed to walls with adhesive blobs ('dot-and-dab') is the fastest method to achieve a skim-ready surface in a renovation
• •Does not create a wet plaster substrate — dries almost immediately
• •Skim is then applied direct to the board ('board finish') using a slightly different mix
• •Disadvantage: the 12–25mm void between the board and the masonry wall allows air movement and can harbour condensation — not recommended for external walls without insulated board

• **Lime plaster (traditional)**:
• •Required for listed buildings and many conservation area properties — applying modern gypsum to a lime-plastered old property traps moisture and causes lime substrate failure
• •Three-coat system: NHL (natural hydraulic lime) render scratch coat, NHL float coat, finish coat with gauged lime putty
• •Substantially slower to dry than gypsum — each coat may need 2–4 weeks before the next is applied
• •Specialist skill — most modern plasterers do not have lime plastering experience; always check
• •Cost: 2–3× the cost of equivalent gypsum work

**New plaster painting sequence**:

1. **Allow drying time**: New gypsum plaster must dry for a minimum of 4–6 weeks before applying standard emulsion. The exact time depends on wall thickness, coat count, ventilation, and season. Winter renovations with poor ventilation take much longer.

2. **Mist coat (diluted emulsion)**: The first coat on new plaster must be a mist coat — standard emulsion diluted with 10–20% clean water (the proportion depends on plaster porosity). The mist coat is absorbed into the surface without 'sealing' it too quickly. Using undiluted paint on new plaster creates a skin that blisters and peels as moisture continues to vapourise underneath.

3. **First full coat**: A full, undiluted emulsion coat applied after the mist coat has fully dried (typically 4–6 hours).

4. **Second full coat**: Final coat for full coverage and finish.

• **Paint types and sheen levels**:
• •Matt emulsion: hides surface imperfections best — the standard choice for walls and ceilings in living areas. Lower sheen reflects less light and is more forgiving on an imperfect plaster surface.
• •Silk/satin emulsion: more washable, used in kitchens, bathrooms, and hallways. More reflective — shows every defect in the underlying plaster more clearly. Only appropriate on a very flat, well-prepared surface.
• •Eggshell (oil-based or water-based): used on woodwork, radiators, and occasionally feature walls. More durable and washable than emulsion.

• **Common painting mistakes**:
• •Painting too soon (moist plaster): causes discolouration, staining, and paint failure
• •Skipping the mist coat: causes blistering and peeling within months
• •Painting silicon/acrylic fillers over unpainted plaster without misting first: the filler seals the surface locally, creating visible bands of different absorption
• •Using silk emulsion on a fresh skim: every trowel mark and imperfection is amplified
• •Not priming bare woodwork before eggshell: poor adhesion and grain raising

• **Skimming over existing surfaces**:
• Old plaster can sometimes be overskim-coated rather than hacked off and replaced, if:
• •The existing plaster is sound and firmly bonded (tap test — hollow sections must be cut back)
• •The surface is free of wallpaper, flaky paint, or oil-based coatings
• •It is stabilised with a bonding agent (PVA diluted 3:1) before skim
• •The existing plaster is not lime — lime and modern gypsum skim are incompatible

**Plastering costs (London 2025)**:

Pricing for plastering is typically per m² for re-skim or new work.

• •Skim coat (re-skim existing wall, plaster sound): £10–£16/m²
• •Skim coat on plasterboard (board finish): £8–£14/m²
• •Full replaster (hack off, hardwall, skim): £18–£28/m²
• •Dot-and-dab plasterboard plus skim: £22–£35/m² (supply and fix, including plasterboard)
• •Lime plastering (three-coat): £45–£75/m² (specialist contractor, not standard plasterer rates)

• For a typical 3-bedroom Victorian terrace full renovation (100–120m² of wall area):
• •Full replaster: £1,800–£3,360 (labour only)
• •Dot-and-dab new partition walls plus skim: £2,200–£4,200 (supply and labour)

**Decorating costs (London 2025)**:

• •Mist coat plus 2 full coats emulsion — walls: £8–£14/m² (labour only; client supplies paint)
• •Mist coat plus 2 full coats emulsion — walls: £12–£20/m² (supply and labour, mid-range paint)
• •Ceilings: £10–£16/m² (supply and labour, mid-range paint)
• •Woodwork (doors, skirtings, architraves — gloss or eggshell): £40–£80 per door set; £8–£15/m run for skirtings
• •Full decoration, 3-bedroom terrace (walls, ceilings, woodwork): typically £5,000–£10,000 supply and labour (mid-range specification)

**Second-fix reminder**: In RCB's standard estimating position, second-fix materials — including paint, tiles, flooring, and fixtures — are client-supplied. Paint specification and finish selection are the client's choice and responsibility; the decorator's role is to apply what is supplied to the agreed specification.