Powder Coating for Aluminium Extrusions and Architectural Work

Architectural aluminium is the most demanding common application for powder coating in India, and the one where specification errors cost the most. A window system, a curtain wall or a louvre installation has a design life measured in decades, sits in full sun, and is visible to the end client every day. Get the chemistry wrong and the failure is public.

What makes extrusions different

Three things separate aluminium extrusion work from general fabrication.

• The substrate is aluminium, not steel. It does not rust, but it does oxidise, and it needs a completely different pre-treatment route. Zinc phosphate is a steel process; aluminium requires a chromate or, increasingly, a chrome-free conversion coating.
• The parts are long and thin. A six-metre extrusion has to hang without sagging, heat evenly and coat consistently along its length. Film thickness variation from one end to the other is a common and avoidable defect.
• Exposure is permanent and outdoors. There is no indoor variant of a window frame. The chemistry decision is made for you.

The chemistry decision

For architectural aluminium the answer is a pure polyester or, where finish quality and design life justify the cost, a polyurethane. An epoxy polyester hybrid is not acceptable on any exterior aluminium regardless of price, because the epoxy backbone chalks under ultraviolet light. The mechanism is set out in fading and chalking.

Oracle publishes accelerated weathering only for Pure Polyester, because that is the only chemistry a report covers. Where an ASTM G154 figure is a contractual requirement, specify Pure Polyester.

Pre-treatment for aluminium

Aluminium forms a natural oxide layer immediately on exposure to air. That layer is thin, variable and not a suitable base for coating, so it has to be removed and replaced with a controlled conversion coating.

A typical route is: alkaline degrease, rinse, acid etch or deoxidise, rinse, chromate or chrome-free conversion coating, demineralised rinse, dry. Some specifications add a chrome-free titanium or zirconium stage in place of chromate for environmental compliance, particularly for export work.

Two failure modes are specific to aluminium and worth naming.

• Filiform corrosion. Thread-like corrosion tracks that creep under the coating from a cut edge or a drilled hole, driven by chloride and humidity. It is a pre-treatment failure, not a coating failure, and it is why the conversion stage and the rinse quality matter more on aluminium than on steel.
• Etch marks and variable appearance. Extrusion die lines, weld heat marks and alloy composition variation all read through a smooth gloss finish. On a fabricated, welded assembly this is often a reason to specify a fine texture rather than a smooth high gloss.

The full sequence is covered in pre-treatment before powder coating.

Powder coating or anodising

The two are the standard alternatives for architectural aluminium and they fail differently. Anodising grows a hard oxide out of the metal, so it cannot peel and it retains a metallic appearance, but it reveals every substrate variation and offers a narrow colour range. Powder coating is applied over the surface, hides imperfections, and offers the full RAL range and any gloss level.

On clean, consistent mill-finish extrusion, anodising is a legitimate and sometimes better answer. On a fabricated, welded assembly it is usually the wrong one, because the process is transparent by nature and will show every weld and grind mark. The full comparison is in powder coating vs anodising.

What to write into the specification

An architectural aluminium specification that avoids disputes contains all of the following:

1. Chemistry: pure polyester or polyurethane, stated explicitly, not "outdoor grade".
2. Shade: a RAL number with the edition, or an approved panel reference.
3. Gloss: a target at 60 degrees with a stated tolerance, for example 30 plus or minus 5 GU.
4. Finish: smooth, fine texture or structured.
5. Dry film thickness: a minimum and a maximum, typically 60 microns minimum on architectural work.
6. Pre-treatment: the conversion coating type, and whether chrome-free is required.
7. Test evidence: which reports the supplier is expected to provide, and to which methods.

Note on standards: IS 13871 is India's standard for thermosetting powder coatings, but it is a material standard rather than an architectural one. Where a project specifies architectural aluminium performance, European systems such as QUALICOAT and GSB are the references usually cited, and they impose requirements on the applicator and the pre-treatment line as well as on the powder.

Practical points on the line

• Hang long sections vertically and support them. A six-metre extrusion that sags in the oven cures unevenly along its length.
• Split loads by mass. A heavy extrusion and a light panel in the same load reach cure temperature at very different times.
• Measure film thickness at both ends and the middle. Along-length variation is the most common architectural rejection.
• Order a visible elevation in one batch. Adjacent frames from two batches, each individually within tolerance, can still show a step. See colour matching.

Minimum order and lead time

Oracle's minimum order quantity is 100 kg per shade, which makes a project-specific custom shade realistic on a single elevation rather than only on volume production. Lead times on stock shades are short; ask at enquiry stage for a date on a custom match rather than a range.

Talk to us before the shade is approved

The cheapest time to fix an architectural specification is before the first panel is coated. Tell us the alloy, the exposure, the shade and the gloss you need, and we will tell you which chemistry applies and what evidence we can supply with it.