Painting Masonry Walls in London: Weathershield vs Silicate, New vs Old Render

Painting Masonry in London: Getting the Specification Right

External masonry painting in London is one of the most technically demanding tasks a professional decorator takes on. The combination of London's climate — wet, cool, with significant freeze-thaw cycling in winter — and the diverse range of masonry substrates across the city's housing stock means that a "one product fits all" approach routinely produces poor results: peeling, flaking, efflorescence, moisture entrapment, and rapid paint failure.

This guide explains the key technical factors: the difference between acrylic/resin-based masonry paints (represented by Dulux Weathershield) and mineral silicate paints (represented by Keim), how to assess and prepare different masonry substrates, and how to make the correct product choice for different building types.

Understanding the Two Fundamentally Different Approaches

Film-Forming Paints: Dulux Weathershield and Equivalents

Dulux Weathershield is the dominant product in the residential masonry paint market. It is an acrylic (styrene-acrylic or silicone-modified acrylic) formulation that dries by evaporation to form a flexible, water-resistant film over the masonry surface.

The film-forming approach works by keeping water out. When the paint film is intact and correctly applied, water runs off the surface rather than penetrating the masonry. The film also has a degree of elasticity that allows it to bridge hairline cracks that develop in render or mortar as the building moves seasonally.

Advantages:

• Easy to apply by brush, roller or spray
• Available in a very wide colour range
• Fast drying (2–4 hours)
• Good initial water resistance
• Widely available at trade and retail

Limitations:

• The film reduces vapour permeability — moisture from inside the building cannot easily escape through the wall, which can cause paint failure from the inside out in buildings with high internal humidity or poor insulation
• Algae and organic growth can colonise the surface, particularly in shaded areas or in London's damp climate — acrylic surfaces are relatively hospitable to biological growth compared to silicate paints
• On older, lime-rich or carbonating substrates, the film can prevent natural carbonation and cause moisture entrapment
• When the film fails (which it will after 5–10 years), it fails by cracking and peeling rather than slowly fading — the maintenance window is narrower

Best suited to: Modern Portland cement render in sound condition; concrete blockwork; painted or rendered non-listed modern buildings; properties where ease of application and cost are the primary drivers.

Main products in this category:

• Dulux Weathershield Smooth — the standard domestic masonry paint
• Dulux Weathershield Masonry Paint (textured variants) — for rough or textured render
• Dulux Weathershield Exterior Primer (for bare masonry) — use before topcoats
• Sandtex Fine Texture and Sandtex Ultra Smooth — competitive products widely used by professional decorators as trade alternatives
• Johnstone's Stormshield — professional-grade acrylic masonry paint, often used by larger decorating firms

Mineral Silicate Paints: Keim and Equivalents

Keim (Keimfarben) is a German manufacturer whose silicate masonry paints are the benchmark of the heritage and conservation painting market. Silicate paints are based on potassium silicate — also known as liquid glass or waterglass — which reacts chemically with the silica and calcium content of the masonry substrate in a process called petrifaction or silicification.

Unlike a film-forming paint, a silicate paint does not sit on top of the masonry — it bonds with it at a molecular level. The result is a surface that is not a film at all, but a mineralised extension of the masonry itself.

Advantages:

• Exceptional breathability — moisture from inside the wall passes freely through the coating. This is critical on older buildings with solid masonry construction where vapour permeability is essential to wall health.
• Extreme durability — silicate surfaces on well-maintained historic buildings in Germany are documented to last 40–80 years without failure. In London's climate, 20–30 years is realistic between full redecoration.
• No peeling or flaking — when the surface eventually weathers, it does so by slow, even chalking rather than film failure
• Excellent resistance to biological growth (algae, lichens, moss) due to the highly alkaline, non-porous mineralised surface
• Appropriate for use on listed buildings — widely accepted by Historic England and conservation officers as compatible with lime render, limestone and historic masonry

Limitations:

• Can only be used on mineral-based substrates — concrete, lime render, lime plaster, limestone, brick. Cannot be applied over modern acrylic paint (the silicate cannot react through the film).
• Must be applied by an applicator who understands the product — technique matters more than with standard masonry paints
• More expensive per litre and requires more careful surface preparation
• Limited colour range (though Keim's range has expanded significantly and now includes several hundred tones)
• Pot life of the mixed product is limited — mixing must be done carefully

Best suited to: Listed buildings; buildings in Westminster, RBKC, Camden or Islington conservation areas with lime render; historic stucco facades; any building where breathability is a priority and durability over a long period is valued.

Main products in this category:

• Keim Granital — the core exterior silicate paint. Available in over 700 colours. The product of choice for most London heritage masonry projects.
• Keim Soldalit — a sol-silicate formulation for slightly wider application, including some surfaces where pure silicate has limitations
• Keim Biosil — specifically formulated for biologically active surfaces with existing algae or moss contamination
• Beeck Mineral Paints — a Keim-equivalent German silicate manufacturer, slightly less well known in the UK market but used by some specialist conservation painters
• Auro Silicate Paint — an eco-focused silicate formulation available in the UK market

Understanding the Substrate: The Critical First Step

The most important factor in specifying a masonry paint system is understanding what substrate you are painting. Getting this wrong — particularly by applying a film-forming paint to an old lime render that needs to breathe — is a common and costly mistake.

New Portland Cement Render

Modern cement render (typically applied since the 1960s) is a high-Portland-cement mix (1:3 to 1:5 cement:sharp sand) that is dense, hard and relatively impermeable. It is:

• High alkaline when new — pH 12–13 immediately after application, which causes saponification of oil-based paints (the alkali breaks down the oil binder). Allow new cement render a minimum of four weeks to carbonate before applying any paint. Six to eight weeks is safer, particularly in cool or wet weather.
• Well-suited to either acrylic masonry paints (Weathershield) or silicate paints (Keim) once fully cured
• Compatible with stabilising primers and alkali-resistant primers before topcoating

Specification for new Portland cement render:

1. Allow full carbonation (6+ weeks)
2. Apply one coat Dulux Weathershield Primer and Sealer or Keim Fixative (for silicate systems)
3. Apply two topcoats of chosen system

Old Lime Render (Pre-1920s Properties)

Victorian, Edwardian and Georgian buildings in London were rendered with lime-based mortars and renders — typically a mix of hydraulic lime or lime putty with sand and sometimes organic fibres. This material is fundamentally different from Portland cement render:

• Breathable — lime render is highly vapour-permeable and this is essential to the thermal performance and structural health of solid-walled buildings. Film-forming paints applied to lime render can trap moisture, which then causes the render to deteriorate and the paint to fail from the inside.
• Soft and flexible — lime render is softer than cement render and moves gently with the building. Film-forming paints that are less flexible can crack and allow water ingress at the crack, which then has no means of escape.
• Alkaline, but less so than fresh cement — old lime render has a lower pH than fresh cement and is generally fine for silicate paint application

Specification for old lime render: Almost always a silicate (Keim) or breathable mineral system. Never apply a dense acrylic masonry paint.

Sound Existing Acrylic Paint (Repaint of an Existing Film-Forming Coating)

If the building already has a sound coat of acrylic masonry paint in reasonable condition, the options are:

• Repaint with the same system (acrylic over acrylic) — the most straightforward approach. Wash, treat any biological growth, fill and stabilise cracks, apply new topcoats.
• Switching to silicate — NOT possible over existing acrylic film. The silicate cannot react with the mineral substrate through the acrylic film. You would need to strip the acrylic paint first (specialist pressure washing plus chemical stripping) before applying silicate.

Application and Timing in London

Weather Conditions

Masonry paint should never be applied:

• When the temperature is below 5°C (acrylic) or below 8°C (silicate)
• When rain is forecast within 4–6 hours for acrylic, 24 hours for silicate
• In direct strong sunlight on a south-facing wall — rapid surface drying causes crazing
• In fog or very high humidity (above 85% RH)

The practical London painting season for exterior masonry is late April through to late September, with the best conditions typically in May, June and September. October and November can work in a mild year but the risks of cold and rain increase substantially.

Preparation is Mandatory

Regardless of which system is chosen, preparation is the most important determinant of longevity. Essential preparation steps:

1. Biocide treatment — apply an exterior fungicide/biocide (Dulux Weathershield Fungicide Treatment, Keim Biosil Pre-Treatment, or equivalent) to any area showing biological growth. Allow 24–48 hours before washing.
2. Pressure washing or scrubbing — remove loose paint, dirt, efflorescence and dead biological matter. Allow to dry fully — minimum one week.
3. Repair cracks — rake out and re-render any cracks wider than 0.5mm. For hairline cracks in cement render, a stabilising primer may bridge them adequately.
4. Stabilise — apply a stabilising primer (Dulux Weathershield Stabilising Primer) to any areas of friable, chalky or powdery render surface.

Key London Planning Considerations

For any masonry painting on a listed building or within a conservation area:

• Westminster City Council — contact the Duty Planner for informal pre-application advice. Any change of colour on the exterior of a listed building requires Listed Building Consent.
• RBKC — similar requirements. The Kensington and Chelsea conservation area appraisals specify preferred colour palettes for several areas. Check the relevant appraisal before specifying a colour.
• Islington — Islington has detailed conservation area design guidelines. The Barnsbury, Canonbury and Highbury conservation areas all have specific guidance on acceptable masonry colours.
• Camden — Camden's conservation officers have experience with silicate paint systems and are generally supportive of Keim as a material on historic buildings.

For unlisted buildings in conservation areas, repainting in the same colour is generally permitted development. Changing colour may require consultation.

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For a professional masonry paint specification and quotation for your London property, our team carries out full substrate assessments before recommending a product. Contact us here or request a free quote. We work with Keim, Dulux Trade, Sandtex and specialist heritage products across Westminster, RBKC, Camden, Islington and beyond.