Painting Crittall and Steel Windows in London: Preparation, Primers and Conservation Considerations

Why Steel Windows Demand a Different Approach

Crittall windows — and the wider category of steel-framed windows common in 1920s–1960s London buildings — are a recurring decorating challenge. The combination of thin metal sections, historical paint layers, exposure to weather and (in many cases) decades of under-maintenance creates a preparation problem that must be addressed properly before any topcoat is applied.

Steel corrodes. Paint systems on steel windows fail at a different rate and in different ways from paint on timber. Understanding the substrate, the failure modes and the correct product specification is the difference between a paint job that lasts fifteen years and one that begins failing within two.

Identifying What You Have

True Crittall windows — made by the Crittall Manufacturing Company, still in production — have slender hot-rolled steel sections with characteristic joinery profiles. They appear across a huge range of London buildings: 1930s Art Deco mansion blocks, modernist houses, industrial conversions and contemporary new-builds (Crittall has had a significant revival since the 2010s). They are also found extensively in basement conversions.

Older steel windows (pre-1960s) were typically primed with red lead primer at the factory, which is highly effective at preventing rust but may not have been maintained adequately since. These windows often show rust bleed at joints, at the glazing rebate and at any point where the paint film has been damaged.

Preparation: The Critical Stage

Strip all loose and flaking paint. A wire brush attachment on a drill, a needle gun or a detail sander (Festool Delta for corners) removes loose material efficiently. Do not apply any primer or topcoat over loose paint — it will bridge and peel.

Address all rust. Surface rust — red-brown discolouration without pitting — can be treated chemically with Hammerite Kurust or Rust-Oleum Rust Reformer. Apply, allow to convert (30–60 minutes), then wipe off any residue. Deeper corrosion with pitting must be mechanically removed to bare metal, ideally with an angle grinder with a flap disc. Once bare metal is exposed, prime the same day — flash rust develops within hours in a London environment.

Clean the entire frame with a solvent wipe (white spirit or Hammerite Brush Cleaner) to remove any grease, silicone or contamination. Allow to dry fully.

Priming

For steel windows, the primer is not optional and not interchangeable with a general-purpose primer.

Rust-Oleum 3369 Metal Primer. A reliable anti-corrosion primer for steel that is compatible with most topcoat systems. Water-based, fast-drying and low-odour — practical for internal work or occupied properties.

Zinsser Bulls Eye 1-2-3. An excellent multi-surface primer that bonds well to bare metal and previously painted steel. Good choice for windows that have been stripped but show no active rust.

Hammerite Direct to Rust Primer. Oil-based, designed specifically for rusty or marginally prepared metal. Use this where complete rust removal is not practical — it penetrates and inhibits rust rather than requiring a perfectly clean surface.

Tor Isoprime. A professional two-pack epoxy primer used by specialist metal window restorers. Exceptional adhesion and corrosion resistance. Higher cost and requires careful mixing; best suited to full restoration projects where longevity is the priority.

Apply primer generously to the glazing rebate, corners and joints where water collects. These are the highest-risk areas and are often under-primed.

Topcoat Selection

Dulux Trade Weathershield Gloss. A conventional oil-based gloss that provides a hard, durable film. Compatible with most metal primers. Good flow and levelling. The standard choice for most London decorators working on steel windows.

Hammerite Smooth. Often used directly on metal as both primer and topcoat. Not recommended over previous paint systems — it is designed for bare metal application. Finish quality is acceptable but not as fine as a separate primer and topcoat system.

Little Greene Intelligent Gloss / Exterior Gloss. Water-based gloss that performs well on steel windows in sheltered positions. Not recommended for fully exposed, unshielded facades. Excellent colour range — useful when matching window colour to a wider decorative scheme.

Zinsser AllCoat Exterior Satin. A versatile water-based finish that adheres to almost anything, tolerates temperature variation and provides a mid-sheen finish. Works well on steel windows in sheltered positions.

Colour Options

Black is the classic Crittall colour and for good reason — it emphasises the slenderness of the sections and reads as a graphic element in the elevation. RAL 9005 (Jet Black) is the closest to the original Crittall specification. Farrow & Ball Off-Black, Little Greene Obsidian Green and Dulux Heritage DH Ebony are popular near-black alternatives that add warmth.

Other popular choices: dark bronze (Farrow & Ball Brinjal, Little Greene Dark Lead), anthracite grey (RAL 7016), forest green. White and off-white remain the most common in conversions and basement flats where natural light is precious.

Conservation Area Considerations

In RBKC, Westminster and Camden conservation areas, Crittall and steel windows are frequently subject to Article 4 directions that restrict or require consent for replacement. Repainting an existing frame in a similar or matching colour is generally permitted without consent. Changing the colour significantly, or replacing steel windows with a different material or profile, may require planning permission.

Before replacing any steel window in a conservation area, check with the relevant LPA. Original Crittall windows that are restorable should be retained and repainted rather than replaced — they are a heritage asset.

Get a Quote

For steel and Crittall window painting in London — restoration, full repaints or as part of a wider exterior scheme — contact us or request a free quote.