Painting · Anodizing · Galvanizing

Surface finishing protects aluminium profiles from corrosion and weathering while providing the visual identity of the building envelope. The three principal processes — electrostatic powder coating, anodizing and galvanizing — each deliver a distinct combination of durability, appearance and cost. The choice depends on the application environment, the desired finish and the service life expected from the system.

Electrostatic powder coating

Powder coating applies a dry, free-flowing pigmented powder to the aluminium surface using an electrostatic spray gun. The charged powder adheres to the grounded profile and is then cured in an oven, where it melts and flows into a continuous, hard film. The result is a tough, decorative coating available in the full RAL range as well as custom colours, metallic shades and textured or matt finishes.

Key characteristics:

• Wide colour and effect range, including textured, anti-bacterial and self-cleaning finishes.
• Low VOC emissions; oversprayed powder can be recovered and reused, making the process environmentally efficient.
• Coating thickness typically 60–120 µm, providing strong barrier protection against weathering.
• Qualicoat certification is the recognised quality benchmark for architectural powder-coated aluminium.

The process consists of three stages: chemical pre-treatment (degreasing, etching, chromate or chrome-free conversion coating), electrostatic powder application, and oven curing at approximately 200 °C for 10 minutes. Proper pre-treatment is critical — without it, filiform corrosion can develop beneath the coating.

Anodizing

Anodizing is an electrochemical process that thickens the natural aluminium oxide layer to form a hard, integral protective film. The profile is immersed as the anode in an acidic electrolyte, and a direct current grows an oxide layer of 5–30 µm on its surface. Because the layer is part of the metal itself, it cannot peel or flake.

Key characteristics:

• Preserves the metallic appearance of aluminium; available in natural, bronze, gold and black tones (per DIN 17611 / EURAS colour notations).
• UV-stable — colour does not fade under sunlight.
• Highly resistant to abrasion and physical contact; suitable for handrails and reachable surfaces.
• Compatible with structural silicone glazing, where adhesion is critical.
• Qualanod certification is the architectural quality standard.

A 20 µm layer is typically specified for architectural use (DIN 17611), while BS standards call for 25 µm. At 25 µm, an anodized coating typically lasts 30–40 years depending on atmospheric conditions, with erosion rates of 0.07–0.50 µm per year.

Galvanizing

Galvanizing applies a zinc coating to steel components used alongside aluminium systems — anchors, brackets, fixings and substructure elements. Although not applied to aluminium itself, it is essential to the long-term performance of the overall facade or fenestration assembly, since corroded fixings can compromise even the most durable aluminium profile.

Two principal methods are used:

• Hot-dip galvanizing: the steel part is immersed in molten zinc at 450–455 °C, producing a metallurgically bonded zinc-iron alloy coating. Typical service life is 30+ years with no maintenance.
• Electro-galvanizing: zinc is deposited electrolytically, producing a thinner, more uniform coating suitable for visible or precision components.

When galvanized steel is subsequently painted (duplex coating), the synergy between the zinc layer and the organic topcoat extends total protection well beyond the sum of the two systems individually, while also enabling colour matching with adjacent aluminium elements.

Selecting the right finish

The table below summarises the principal differences between powder coating and anodizing for architectural aluminium. Galvanizing is included for completeness in respect of steel sub-components.

Property	Powder coating	Anodizing	Galvanizing
Substrate	Aluminium	Aluminium	Steel
Appearance	Opaque, full colour range	Translucent, metallic finish	Matt grey, utilitarian
Colour fastness	Very good (UV-stable polymers)	Excellent (inorganic)	N/A (natural zinc)
Abrasion resistance	Good	Excellent	Good
Typical thickness	60–120 µm	15–25 µm	60–85 µm (hot-dip)
Service life	20–25 years	30–40 years	30+ years
Certification	Qualicoat	Qualanod	EN ISO 1461

Wood-effect (also called wood-grain or sublimation) finishing transfers a realistic timber pattern onto powder-coated aluminium profiles. The result combines the warm visual character of wood with the durability, dimensional stability and low-maintenance properties of aluminium — a finish particularly suited to windows, doors and facade cladding where natural appearance is desired without the upkeep of real timber.

Process

The finish is applied in two stages. First, the profile receives a base powder coat in a brown tone that sets the underlying shade (light oak, dark walnut, mahogany, etc.). Second, a wood-grain transfer film is wrapped around the profile, sealed at both ends under vacuum and passed through an oven. The heat sublimates the pattern from the film into the powder coat, producing a permanent decorative layer that follows the full geometry of the profile, including grooves and recesses.

Design freedom

Because the base coat and transfer film are independently variable, the available combinations are extensive — any base tone can be paired with any grain pattern. In thermally broken systems, the interior and exterior faces of the profile can carry different finishes, allowing an anodized exterior with a wood-effect interior, or two different wood tones on the same window.

• Available patterns: oak, walnut, mahogany, pine, cherry, teak and custom designs.
• Full UV stability — no fading or yellowing under prolonged sun exposure.
• Same maintenance requirements as standard powder coating — wipe clean with water.
• Qualideco certification is the recognised quality standard for decorative finishes on aluminium.

The quality and durability of any surface finish depend on the pre-treatment applied to the bare aluminium before coating. Pre-treatment removes contaminants from extrusion, etches the surface to create a mechanical key, and applies a conversion layer that promotes coating adhesion and provides a first line of corrosion protection. Inadequate pre-treatment is the single most common cause of premature coating failure.

Chemical pre-treatment stages

• Degreasing: removes oils, drawing compounds and handling residues from the extrusion process.
• Alkaline or acid etching: dissolves the native oxide layer and produces a uniform, micro-roughened surface.
• Desmutting: removes alloy-element residues (silicon, copper, manganese) left behind after etching.
• Rinsing: multiple demineralised water rinses between chemical stages prevent cross-contamination.
• Conversion coating: deposits a thin inorganic film — historically chromate, now increasingly chrome-free chemistries based on titanium, zirconium or silanes — that bonds the coating to the metal.

Chrome-free chemistries

Hexavalent chromium has been progressively phased out under REACH and similar regulations due to its toxicity. Modern chrome-free conversion coatings based on titanium-zirconium fluoride or organo-silane chemistries deliver equivalent corrosion resistance and coating adhesion while eliminating the environmental burden. Qualicoat Seaside specification, in particular, recognises these systems for coastal exposure conditions.

Mechanical pre-treatment

Mechanical processes — abrasive blasting, brushing or polishing — are applied before chemical pre-treatment when a specific surface texture is required. Brushing with stainless-steel wheels produces a satin (matt) finish that masks extrusion lines, while polishing with liquid or solid compounds yields a bright finish. These textures are then preserved by the subsequent anodizing or coating process.

Aluminium is naturally corrosion-resistant because of its self-forming oxide layer, but this passive protection can be compromised in aggressive environments — coastal, industrial or where dissimilar metals are in contact. Understanding the mechanisms of corrosion guides the choice of finish, fixing materials and detail design.

Types of corrosion to consider

• Filiform corrosion: thread-like attack under organic coatings, triggered by inadequate pre-treatment and ingress of moisture.
• Pitting corrosion: localised attack in chloride-rich environments (coastal sites, swimming pools, de-icing salt).
• Galvanic corrosion: occurs where aluminium contacts a more noble metal (copper, stainless steel) in the presence of an electrolyte; prevented by isolating the contact with EPDM, nylon or polyamide spacers.
• Crevice corrosion: develops in stagnant water trapped in joints, gaps or under fixings; prevented by drainage detailing and avoiding water traps.

Exposure categories

EN ISO 12944 defines corrosivity categories from C1 (low — heated interiors) to CX (extreme — offshore, chemical plant). Architectural aluminium is most commonly specified for C3 (urban) to C5 (coastal/industrial) environments. Qualicoat Seaside and Qualanod for marine atmospheres are the relevant performance specifications above C3.

Independent quality marks provide assurance that surface-finishing operations meet defined performance standards. Specifying certified products protects the building owner against premature finish failure and supports warranty claims. The principal certifications encountered in architectural aluminium are listed below.

Certification	Scope	Issuing body
Qualicoat	Powder coating on aluminium for architectural use	Qualicoat (international)
Qualicoat Seaside	Powder coating for coastal / marine exposure	Qualicoat (international)
Qualanod	Anodizing on aluminium for architectural use	Qualanod (international)
Qualideco	Decorative coatings (wood-effect, sublimation) on aluminium	Qualideco (international)
GSB International	Powder coating and anodizing — German quality association	GSB International
EN ISO 1461	Hot-dip galvanizing on steel articles	ISO
EN ISO 12944	Corrosion protection of steel by paint systems	ISO

Routine cleaning preserves the appearance and prolongs the service life of all aluminium finishes. Atmospheric pollutants, salt spray and traffic film accumulate on exposed surfaces and, if left, can locally degrade the protective layer. A consistent maintenance regime is more cost-effective than reactive repair, and is typically a condition of manufacturer warranties.

Recommended cleaning intervals

• Rural / low-pollution environments: once per year.
• Urban environments: every six months.
• Industrial or coastal environments: every three months, or more frequently if salt deposits are visible.
• Swimming-pool environments: monthly inspection and cleaning.

Cleaning methods

Use a soft cloth or sponge with warm water (maximum 25 °C) and a pH-neutral detergent (pH 5–8). Rinse thoroughly with clean water and dry with a soft cloth. Avoid abrasive pads, scouring powders, solvent-based cleaners and high-pressure jets, all of which can damage the coating or anodized layer. Strongly acidic or alkaline products must never be used.

Inspection and repair

Annual visual inspection should check for scratches, chips and signs of corrosion. Small damages to powder-coated surfaces can be touched up with manufacturer-supplied repair paint matched to the original RAL reference. Anodized finishes cannot be locally repaired because the oxide layer is integral to the metal — significant damage typically requires component replacement.

Colour and finish are among the earliest decisions in a facade or fenestration project, and they shape both the visual identity of the building and its long-term maintenance profile. The available palette extends well beyond the standard RAL chart — but each option carries practical implications for cost, lead time and weathering behaviour.

Standard RAL colours

The RAL Classic system (213 colours) is the industry baseline for powder-coated aluminium. Most manufacturers hold stock of common architectural shades — RAL 9016 (traffic white), RAL 9005 (jet black), RAL 7016 (anthracite grey), RAL 9006 (white aluminium) — enabling short lead times. Non-stock RAL colours are produced to order with a typical surcharge and a 2–4 week extension to delivery.

Metallic and special finishes

• Metallic powders (RAL Effect, Interpon D Anodic, Tiger Drylac series) replicate the appearance of anodized aluminium with the colour range of powder coating.
• Textured and fine-structure finishes mask minor surface imperfections and reduce perceived reflectivity — favoured for large facade panels.
• Matt finishes (gloss level 5–30) are increasingly specified for contemporary architecture; high-gloss (>80) finishes show surface defects more readily.
• Anti-graffiti and self-cleaning powders use modified surface chemistry to resist marker pens and shed dust with rainfall.

Dark colours and solar gain

Dark colours (light reflectance value below 25%) absorb more solar radiation and can reach surface temperatures of 70–80 °C in direct summer sun. This thermal load is significant for thermally broken profiles: the polyamide thermal break and the EPDM gaskets must be rated for the expected service temperature, and the system manufacturer's guidance on maximum panel size for dark colours must be respected to avoid bow or seal damage. Cool-pigment formulations are available for very dark shades and can lower peak surface temperature by 10–15 °C.

Surface-finishing processes are increasingly assessed for their environmental footprint as part of overall building lifecycle analysis. Both powder coating and anodizing have made substantial progress in reducing emissions, waste and energy intensity over the past two decades.

Powder coating

• Near-zero VOC emissions — no solvent carrier is used.
• Overspray recovery typically exceeds 95%, with reclaimed powder reused in subsequent batches.
• No hazardous liquid waste; conversion-coating effluent is the principal waste stream and is increasingly closed-loop.
• Curing oven energy is the dominant operational input; modern gas catalytic and infrared systems reduce consumption by 20–30%.

Anodizing

• No organic chemicals or solvents; the process uses sulfuric acid electrolyte recovered and recycled in closed-loop systems.
• Aluminium sulfate by-product is sold to water-treatment industries — a net resource flow rather than waste.
• The anodized layer is inert and does not release particles or compounds over service life.
• End-of-life aluminium with anodized finish is fully recyclable without separation.

Environmental Product Declarations

EPDs (Environmental Product Declarations) under EN 15804 are increasingly required for green building certification (LEED, BREEAM, DGNB). Specifying surface finishes from manufacturers with verified EPDs supports project-level carbon accounting and qualifies for credit under most certification schemes.

The RAL Classic system is the international standard for powder-coated aluminium colours. Each shade is identified by a four-digit code: the first digit denotes the colour family (1xxx yellow, 2xxx orange, 3xxx red, 4xxx violet, 5xxx blue, 6xxx green, 7xxx grey, 8xxx brown, 9xxx white/black). The selection below shows the most commonly specified architectural shades; the full 213-colour range is available on request, and special colours can be matched to NCS, Pantone or sample references.

Note: printed and screen reproductions of RAL colours are indicative only. A physical sample panel produced by the powder-coating facility must be approved before production for any colour-critical application.

Anodized aluminium finishes are classified under two parallel naming systems. DIN 17611 designates pre-treatment with E0–E6 codes and colour with EV1–EV6 codes (e.g. E6/EV1 = chemically matted + natural anodized). The European Anodisers Association (EURAS) uses C0 for natural finish and C31–C35 for bronze and black tones. Unlike RAL, anodized colours have no universal hex specification — the appearance depends on the alloy, pre-treatment and electrolyte composition. The hex values shown below are typical visual approximations and should be confirmed against a physical sample panel.

The full anodizing palette is narrower than powder coating: natural silver, three to five bronze tones, gold and black are the architectural staples. The metallic character of the substrate remains visible through the translucent oxide layer in all cases.

DIN 17611 pre-treatment codes

Pre-treatment is specified separately from colour in DIN 17611. The two codes are combined (e.g. E6/EV1) to fully describe an anodized finish.

Code	Pre-treatment	Resulting appearance
E0	Degreasing and oxidation only	Extrusion lines visible
E1	Grinding	Uniform matt, no extrusion lines
E2	Brushing	Satin texture along brush direction
E3	Mechanical polishing	Bright, mirror-like
E4	Grinding + brushing	Fine satin, uniform
E5	Grinding + polishing	High gloss with depth
E6	Chemical matting	Uniform soft matt, no directional texture