Painting Aluminium Windows and Doors in London: Etch Primer, Adhesion and RAL Colour Matching

Why Aluminium Needs a Different Approach

Aluminium windows and doors are increasingly common in London properties — from the slim-profile Crittall-style frames in converted warehouses in Bermondsey and Battersea to the high-specification thermally broken units in new-build apartments across Nine Elms, Canary Wharf, and the South Bank. They are also found in mid-century properties across London, where original aluminium frames from the 1960s and 1970s are still in service.

Unlike timber, aluminium presents a fundamentally different surface for paint adhesion. It is non-porous, smooth, and forms a thin oxide layer on its surface that, while protective against corrosion, actively resists the bonding of conventional paints. Applying standard primer and topcoat to aluminium without proper preparation will result in peeling — sometimes within weeks.

Surface Preparation: The Foundation of Adhesion

The preparation of aluminium before painting is more critical than with any other substrate. The process involves three stages:

Cleaning — aluminium frames accumulate a film of oxidation, dirt, and silicone residue (from sealants and weatherproofing). This must be removed completely. A dedicated aluminium cleaner or a solvent wipe with white spirit or isopropyl alcohol is essential. Household cleaners are insufficient — many contain alkalis that react with aluminium and compromise the surface.

Abrading — the smooth surface must be given a mechanical key. Fine abrasive pads (Scotch-Brite or equivalent) or 240-grit sandpaper are used to scuff the entire surface evenly. The goal is not to remove material but to create micro-scratches that the primer can grip. Power sanding is generally too aggressive and risks distorting the thin aluminium profiles.

Degreasing — after abrading, the surface must be wiped clean of all dust and residual grease with a tack cloth and a final solvent wipe. Any contamination left on the surface will prevent the primer from bonding.

Etch Primer: The Critical Layer

An etch primer — also called a wash primer or self-etch primer — is the key product in any aluminium painting system. It contains phosphoric acid, which reacts chemically with the aluminium surface, converting the oxide layer into a stable phosphate film that provides an excellent mechanical and chemical bond for subsequent coats.

Professional-grade etch primers for aluminium include Rust-Oleum's Metal Primer, Teamac's Metalastic Etch Primer, and Sherwin-Williams' Wash Primer. These are typically applied in a single thin coat — over-application reduces adhesion rather than improving it — and allowed to flash off for the specified time before overcoating.

In London, where much aluminium painting is carried out on occupied residential properties, the choice between solvent-based and water-based etch primers is increasingly relevant. Solvent-based products remain the professional standard for adhesion, but low-VOC alternatives are improving and may be preferred where ventilation is limited or residents are in occupation.

Topcoat Systems: Achieving a Factory Finish

After the etch primer, the topcoat system determines the final appearance and durability:

Direct-to-metal (DTM) acrylic paints — products like Zinsser AllCoat Exterior or Tikkurila Panssarimaali — are formulated to bond to primed metal surfaces and provide a smooth, even finish. They dry quickly, have low odour, and are available in a wide colour range. For most residential aluminium painting in London, a DTM acrylic over an etch primer is the standard specification.

Two-pack polyurethane systems — used in commercial and high-specification residential work, these provide a finish virtually indistinguishable from factory-applied powder coating. They are exceptionally durable and colour-stable but require careful mixing, controlled application conditions, and professional spray equipment. The finish is superb but the cost and complexity are higher.

Spray application — regardless of the product chosen, aluminium windows and doors should almost always be sprayed rather than brushed or rolled. The thin aluminium profiles show brush marks readily, and the smooth factory finish that clients expect can only be replicated with a fine-finish spray gun. HVLP (high volume, low pressure) equipment is standard, and careful masking of glass and surrounding surfaces is essential.

RAL Colour Matching

Aluminium windows and doors are typically specified using the RAL colour system — a standardised colour reference used across European manufacturing. The most common RAL colours for London windows are:

• RAL 7016 (Anthracite Grey) — the dominant choice for contemporary slim-profile frames
• RAL 9005 (Jet Black) — popular for Crittall-style and industrial aesthetic frames
• RAL 7021 (Black Grey) — a softer alternative to jet black
• RAL 9010 (Pure White) and RAL 9016 (Traffic White) — for frames in traditional or heritage settings

When repainting existing aluminium frames, matching the original RAL colour precisely is important for consistency — particularly in apartment blocks or terraces where multiple units share the same frame specification. A spectrophotometer reading of the existing colour, taken from an area protected from UV fading, will identify the original RAL code.

Common Mistakes to Avoid

Several errors recur in aluminium painting projects across London:

• Skipping the etch primer — no amount of sanding will compensate for the absence of a chemical bond. The paint will fail.
• Painting in damp conditions — aluminium condenses moisture readily. If the surface temperature is within three degrees of the dew point, do not paint. In London's variable weather, early morning condensation on aluminium frames is common and must be allowed to evaporate.
• Using masonry or general-purpose primers — these are not formulated for non-ferrous metals and will not provide adequate adhesion.
• Insufficient masking — overspray on glass, brickwork, or surrounding render is difficult to remove and detracts from an otherwise professional result.

Maintenance and Longevity

A properly prepared and painted aluminium frame in London should last eight to twelve years before requiring maintenance, depending on exposure and orientation. South-facing frames in direct sunlight will fade sooner; sheltered frames may last longer. Regular cleaning with a non-abrasive solution extends the life of the coating and maintains its appearance.

When repainting becomes necessary, the process is simpler the second time — provided the original system was correctly applied. A light abrade, a clean, and a fresh topcoat are usually sufficient without returning to bare metal.