Painting Concrete Render Exteriors in London

Concrete Render as an Exterior Substrate

A significant portion of London's housing stock — particularly post-war blocks, 1960s and 1970s flat conversions, and some inter-war terraces — has exterior walls finished in concrete render. It differs from lime render or traditional stucco in important ways: it is harder and less breathable, it cures to higher alkalinity, and it cracks in a distinctive and predictable pattern under thermal movement.

Painting concrete render is not the same as painting brick or lime stucco. Using the wrong products or skipping preparation stages leads to rapid failure — blistering, peeling, and paint bridges that mask ongoing deterioration beneath. Done correctly, a properly painted and maintained concrete render exterior should last eight to twelve years before it needs repainting.

Surface Preparation

Concrete render preparation starts with a thorough inspection. Look for:

Active cracks. Hairline cracks (under 0.3 mm) may be stable; wider cracks or those that follow the pattern of the underlying blockwork suggest ongoing movement and need flexible repair rather than filler.

Hollow or delaminating render. Tap the surface with a small hammer — a hollow sound indicates the render is no longer bonded to the substrate beneath. Hollow sections must be cut out and re-rendered; painting over them will fail quickly.

Biological growth. Green algae, black mould, and lichens are common on north-facing and shaded concrete render in London's damp climate. All biological growth must be killed and removed before painting. Apply Dulux Trade Weathershield Fungicidal Wash, leave for 24 hours minimum, then wash down with a pressure washer at low-medium pressure.

Efflorescence. White crystalline deposits on the surface indicate water is migrating through the render and depositing salts as it evaporates. Efflorescence must be brushed off dry — not washed — and any water ingress route identified and resolved before painting.

After cleaning, allow the surface to dry fully. Concrete render can hold significant moisture and should be tested with a moisture meter before priming. Readings above 18–20% WME suggest the surface needs more drying time.

Alkali-Resisting Primer: Non-Negotiable on New Render

Fresh concrete render is highly alkaline — pH 12–13 — and this alkalinity will saponify oil-based paints and can cause adhesion failure in some water-based coatings if priming is skipped. Even on older render, if any patch repairs have been made with cement-based mortar, those areas must be primed with alkali-resisting primer.

Dulux Trade Alkali Resisting Primer applied by brush or roller to a clean, dry surface provides the necessary barrier. Allow the primer to cure for the manufacturer's recommended period — typically 16–24 hours at 20°C, longer in cold or damp London weather.

On older render that is powdery or exhibiting surface breakdown, apply Zinsser Gardz or a stabilising primer first, allow to cure, then apply alkali-resisting primer over it. Two-stage priming adds cost but is justified on any surface with compromised integrity.

Crack Repair Before Topcoating

Concrete render cracks. This is inevitable given London's thermal cycling — summer temperatures above 30°C in recent years followed by cold winters mean exterior surfaces expand and contract repeatedly. The question is not whether to repair cracks but which product to use.

For stable hairline cracks, a flexible exterior filler applied by knife and tooled flush is adequate. Toupret Fibacryl is well-suited to this application — it is fibre-reinforced, weather-resistant, and paintable with both water and solvent-based topcoats.

For wider cracks (1–2 mm and above) or cracks that have opened and closed across seasons, use a flexible crack sealant such as Soudal Fix All or a low-modulus silicone, tooled flush and painted over once fully cured. Note that standard emulsion paints will not bond to silicone — use a silicone masonry topcoat or prime the repair with Zinsser Bulls Eye before applying acrylic topcoat.

Do not use rigid filler compounds such as Polyfilla or standard exterior masonry filler on active cracks. These will simply crack again at the same point.

Silicone vs Acrylic Masonry Topcoats

This is the most consequential specification decision on a concrete render exterior.

Acrylic masonry paint — Dulux Trade Weathershield Smooth, Sandtex Trade Smooth — is widely used, reasonably priced, and provides a durable, UV-resistant finish in a broad colour range. It is water-based, typically 10–15 year durability claim from manufacturers, and suitable for most concrete render applications where the render is in sound condition. Coverage rates are typically 8–10 m² per litre for topcoat.

Silicone masonry paint — Remmers Siliconharz Fassadenfarbe, Sto Stolit K, or Teknos Siltek E-120 — offers superior breathability, water repellency, and self-cleaning properties. Silicone in the binder allows the paint film to shed water (high water contact angle) while allowing water vapour to pass out through the wall. On concrete render in London, where trapped moisture is a genuine concern, silicone masonry paint significantly reduces the risk of blistering and reduces biological regrowth rates between maintenance cycles.

The cost premium for silicone masonry paint is approximately 30–50% over acrylic on materials. On a full exterior repaint with mobilisation and preparation costs factored in, the additional material cost is modest against the longer performance interval.

Colour and Finish Considerations

Concrete render is a relatively flat, relatively smooth substrate. A smooth masonry topcoat in any finish level from flat to satin applies well to it. Textured masonry paint is generally unnecessary unless the render has significant surface variation you want to mask.

Darker colours on south or west-facing elevations in London can accelerate thermal movement and are more likely to show UV bleaching over time. Mid-tones and lighter shades perform better in terms of long-term colour retention, though modern acrylic and silicone binders have improved significantly in UV resistance.

Get a Professional Assessment Before You Paint

Concrete render in poor condition — spalling, hollow sections, widespread cracking — should be assessed structurally before any painting programme begins. We are experienced in working with structural engineers and render contractors where underlying issues need addressing before decoration.

Contact us for a free site survey and quote covering exterior render painting across London SW1, SW3, SW7, and surrounding areas.