How to Plaster Over Brick: Internal and External Methods
Quick Answer: You can absolutely plaster over brick, both internally and externally. For internal walls, use a bonding coat (typically 8-11mm) followed by a 2-3mm skim coat. For external walls, apply a render system in two or three coats totalling 15-25mm. The key is proper preparation: clean the brick, dampen the surface, and use the correct mix ratio. Internal plastering costs £25-45/m² whilst external rendering costs £40-80/m² depending on the finish type. Why Plaster Over Brick? Plastering over brickwork transforms rough masonry into smooth, paintable surfaces ready for decoration. Whether you’re converting an old garage, finishing a new-build extension, or simply want to hide tired brick internally, this technique is fundamental to modern construction. There are several compelling reasons to plaster over brick walls: Aesthetic improvement: Creates a smooth, contemporary finish perfect for paint or wallpaper Thermal efficiency: Adds insulation value (especially with insulated render systems or thermal backing) Weather protection: External rendering shields brickwork from moisture penetration and frost damage Sound dampening: Additional mass reduces noise transmission through walls Damage concealment: Hides cracked, mismatched, or poor-quality brickwork The method you choose depends entirely on whether you’re working internally or externally, the condition of the brick, and the final finish you’re after. Let’s break down both approaches in detail. Internal Plastering: The Two-Coat Method Internal brick walls require a two-coat approach: a base coat (also called an undercoat or floating coat) followed by a finishing coat (skim coat). This traditional method has been refined over decades and remains the standard for achieving durable, smooth internal walls. Materials You’ll Need for Internal Work Material Purpose Coverage Approximate Cost (2026) Bonding plaster (Thistle Bond It or similar) Base coat adhesion to brick 2.6m² per 25kg bag at 11mm £9-12 per 25kg bag Browning plaster Alternative base coat for normal suction surfaces 2.3m² per 25kg bag at 11mm £8-11 per 25kg bag Multi-finish plaster Final skim coat 10m² per 25kg bag at 2mm £8-11 per 25kg bag PVA bonding agent Seals porous surfaces, improves adhesion Varies by dilution ratio £12-18 per 5L container You’ll find these materials readily available at Screwfix, Wickes, or Travis Perkins. British Gypsum products (Thistle range) are the industry standard, though Knauf and Siniat also produce excellent alternatives. Step 1: Preparation and Assessment Before touching a trowel, spend time properly preparing the brick surface. This stage determines whether your plaster lasts decades or fails within months. Clean the brick thoroughly: Remove all dust, loose mortar, efflorescence (white salt deposits), and organic growth. A stiff wire brush works well, followed by a vacuum to extract dust from mortar joints. For stubborn contamination, use a diluted brick acid solution (following manufacturer safety guidelines). Check for dampness: Brick must be dry before plastering. Use a moisture meter to confirm readings below 15%. If the wall shows signs of rising damp, penetrating moisture, or condensation issues, resolve these first. Plastering over damp brick leads to adhesion failure, efflorescence blooming, and potential mould growth. ⚠️ Warning: Never plaster over brick affected by rising damp without first installing a proper damp-proof course (DPC). This typically involves chemical injection or physical membrane installation. Consult a specialist if you suspect serious moisture issues, as per Government guidance on damp and mould. Assess the wall’s flatness: Run a straight edge across the brick surface. Deviations greater than 10mm over 2 metres may require dubbing out (building up low areas) with extra base coat material before the main application. Step 2: Priming the Surface Brick has variable suction rates depending on its age, type, and firing. Proper priming ensures consistent adhesion and prevents the brick from drawing moisture too quickly from your plaster mix. Apply PVA solution (1 part PVA to 3-4 parts water) using a roller or large brush. For very porous brick, apply two coats: the first diluted 1:5, allowing it to dry, then a second coat at 1:3 just before plastering. The surface should feel tacky but not wet when you apply the base coat. Alternatively, for extremely dense engineering bricks or glazed surfaces, use a specialist bonding agent like SBR (Styrene Butadiene Rubber) diluted as per manufacturer instructions. These create a mechanical key on low-suction surfaces. Step 3: Applying the Base Coat The base coat provides thickness, builds out uneven areas, and creates a level surface for the finishing coat. For most internal brick walls, aim for 8-11mm thickness. Mixing bonding plaster: Add plaster to clean water (never water to plaster) in a bucket. For a 25kg bag, use approximately 11-12 litres of water. Mix thoroughly with a paddle mixer until you achieve a thick, creamy consistency that drops slowly from the trowel without running. Professional plasterers often gauge the consistency by feel: when you lift the mixing paddle, the plaster should form peaks that slowly collapse. Too wet and it’ll slump; too dry and it won’t adhere properly or trowel smoothly. Pro Tip: Work in bays of 2-3 square metres at a time if you’re inexperienced. Bonding plaster has a working time of around 90 minutes after mixing, but this reduces as you trowel and re-trowel. Starting with smaller areas prevents wastage and reduces pressure whilst you’re developing your technique. Application technique: Load your laying-on trowel (typically 280mm or 300mm) with plaster from your hawk Apply to the wall with firm, upward strokes at a 30-degree angle Work from bottom to top, slightly overlapping each stroke Build up to your desired thickness (usually achieved in one coat for 8-11mm) Use a derby (darby) or feather edge to flatten and level the surface Fill any hollows with additional material and re-level Once you’ve covered your area and levelled it, allow the plaster to firm up for 30-45 minutes. As it stiffens, you can devil the surface using a scratcher or the teeth of a plasterer’s comb. This creates horizontal grooves that provide a mechanical key for the finishing coat. Allow the base coat to dry thoroughly before skimming. In normal conditions (15-20°C, moderate humidity), this takes 24-48 hours. The surface should be firm to
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