Painting Crittall Windows and Metal Frames in London

The Enduring Appeal of Crittall Windows

Crittall steel windows are one of the defining architectural features of London's interwar and Art Deco buildings. Found in mansion blocks across Mayfair, converted industrial spaces in Battersea and period homes throughout Chelsea, these slender metal frames offer a distinctive aesthetic that modern aluminium or PVC alternatives cannot replicate.

However, steel frames demand a specific maintenance approach. Unlike timber, which absorbs and releases moisture gradually, steel corrodes rapidly once its protective paint system fails. Proper painting is not merely cosmetic — it is essential preservation.

Understanding the Enemy: Corrosion

Steel window frames corrode through a straightforward chemical process: when moisture reaches bare metal in the presence of oxygen, iron oxide (rust) forms. In London's climate, with its combination of rain, humidity and historically acidic pollution, unprotected steel deteriorates quickly.

The challenge with Crittall windows is their construction. The frames are assembled from relatively thin rolled-steel sections with riveted or welded joints. Moisture can penetrate at joints, around glazing putty and through any chip or crack in the paint film. Once corrosion begins, it spreads beneath the paint, causing bubbling and flaking that accelerates the problem.

Assessment Before Painting

Before reaching for a brush, we carry out a thorough assessment of each window:

• Extent of corrosion: Surface rust is easily addressed. Deep pitting or perforation may require metalwork repair by a specialist before painting can proceed.
• Putty condition: Steel-frame putty (metal casement putty) fails over time. Cracked or missing putty must be replaced to prevent water ingress behind the glass.
• Operability: Corroded hinges, stays and fasteners should be freed, lubricated or replaced during the painting process.
• Previous paint system: Knowing what is already on the frames helps determine compatibility. Oil-based systems are most common on older Crittall windows.

Preparation: Removing Rust and Old Paint

Preparation on steel frames follows a different process to timber:

Wire brushing and scraping: Remove all loose and flaking paint using a wire brush and scraper. Work carefully around glazing to avoid cracking or dislodging glass panes.

Rust treatment: Areas of active corrosion must be taken back to bright metal where possible. We use a combination of wire brushes, abrasive pads and rust converter chemicals. Rust converter transforms iron oxide into a stable compound that can be over-painted, and is particularly useful in areas where mechanical removal to bright metal is impractical.

Degreasing: Steel frames can harbour oils and contaminants that prevent paint adhesion. A solvent wipe ensures a clean surface.

Sanding: Sound existing paintwork is keyed with 120-grit abrasive paper to provide a mechanical bond for the new paint system.

The Paint System

A proper paint system for Crittall windows involves distinct layers, each serving a specific function:

1. Rust-inhibiting primer: This is the most important coat. We use a high-quality zinc phosphate or red oxide primer formulated for ferrous metals. The primer must be applied to all bare metal areas on the same day they are prepared — leaving bare steel overnight invites flash rusting.

2. Undercoat: An oil-based undercoat builds film thickness and provides opacity. One or two coats depending on the depth of colour change.

3. Topcoat: A durable exterior gloss or eggshell specifically rated for metal surfaces. We typically apply two topcoats for maximum protection, particularly on exposed elevations.

The total dry film thickness should be sufficient to provide a continuous barrier against moisture. Thin, skimpy coats are a false economy on steel windows.

Application Technique

Crittall windows require a different painting technique to timber:

• Use a small brush: The narrow sections of steel glazing bars demand a 12mm or 19mm brush for precision work.
• Paint overlap onto glass: As with timber sash windows, the paint should extend approximately 2mm onto the glass surface to seal the junction between putty and pane.
• Work systematically: Paint opening casements first (starting with glazing bars, then rails, then stiles), followed by fixed lights, then the main frame.
• Avoid thick coats: Multiple thin coats outperform fewer thick coats on metal. Thick paint on steel sections is prone to sagging and takes longer to cure.

Colour Choices

Traditional Crittall windows were typically painted in dark colours — black, dark green or dark grey. Many listed buildings and conservation areas in Westminster, Kensington and Camden specify approved colours for external metalwork.

We check local planning requirements before agreeing a colour with the homeowner. For properties in Belgravia's conservation area, we liaise with Westminster City Council where necessary to confirm acceptable colour specifications.

Maintenance Schedule

Steel windows should be inspected annually and touched up at the first sign of paint failure. A full repaint cycle of every five to seven years, depending on exposure, will keep Crittall frames in sound condition for decades.

Properties in Fulham and Battersea with south or west-facing elevations may require more frequent attention due to greater UV and weather exposure.

Professional Versus DIY

Painting Crittall windows is technically demanding and physically painstaking. The narrow profiles, the importance of rust treatment and the need for multiple thin coats make this a job best left to experienced decorators. Poorly applied paint on steel frames does not simply look bad — it leads directly to structural deterioration of the window.

Contact us to arrange an assessment of your Crittall windows and receive a detailed quotation for their restoration and repainting.