Powder coating metalwork: what to specify

At a glance

um is micrometres (microns). Corrosivity classes C2 to C5 are the ISO 12944-2 atmospheric classes: C1 is a heated clean interior, which Qualisteelcoat does not use as a column, C4 includes indoor swimming pools. The Qualicoat figures are from a March 2021 comparison that pre-dates the 26th Edition.

Two schemes, one for each metal

Most pages that explain powder coating specifications quote the aluminium standards. Qualicoat, the Swiss-based quality label, describes itself as a product certification scheme for organic coatings on aluminium for architectural applications. It runs through three volumes, for coaters, for chemical pre-treatment and for powder makers, with licences granted and inspected through national associations. BS EN 12206-1:2021, published by BSI on 31 May 2021 and replacing the 2004 edition, is the European standard for coatings from thermosetting powder on aluminium and its alloys. A standard sets the requirements and test methods; a scheme such as Qualicoat adds approvals, a licence and inspections, and lists EN 12206-1 among its test standards. Qualicoat UK & Ireland, the national association, argues that its specification is the more robust of the two, partly because it adds filiform corrosion tests that it says EN 12206-1 does not require. That is an interested party's view, and we have not read the standard's text to check it.

Much bespoke metalwork is steel, not aluminium. The scheme that covers it is Qualisteelcoat, which describes itself as an international quality label for organic coatings on steel and publishes its specification, version 5.0.1, for download, valid from 1 January 2026. Its four base materials are bare steel, continuously galvanised steel, hot-dip galvanised steel and thermally sprayed steel. For hot-dip galvanised steel, BS EN 13438:2013 sets performance requirements for powders and the finished coating but not the coating process, and BS EN 15773:2018 sets out the agreements between the client, the galvanizer, the chemical suppliers and the coater for the combined galvanised and powder-coated system, known as duplex. We found no standard or scheme for powder on stainless steel, brass or copper.

Qualicoat UK & Ireland suggests the specifier writes: “Architectural powder coating to QUALICOAT specification from approved supplier”, and, near the coast, the same with ‘Seaside’ added. It says there is no need to quote an edition, because the specification is revised continually. We would add the class, the colour and the gloss, because the scheme does not choose them for you.

Film thickness: what the schemes ask for, and how it is accepted

On aluminium, a March 2021 comparison published by the Powder Coating Institute lists the Qualicoat classes with a minimum film of 60 um for classes 1 and 2 and 50 um for class 3, measured to ISO 2360, which is a method for non-conductive coatings on non-magnetic metal such as aluminium. Class 3, the most weather-resistant, has the lower minimum. The comparison pre-dates the 26th Edition, so confirm the figure against the current volumes. A trade article reports that the 2021 revision of BS EN 12206-1 sets an absolute minimum of 40 um; we could not read the clause and do not rely on the figure.

On steel, Qualisteelcoat's Annex 3 lists whole systems, not a single thickness. For polyester powder the table gives these, with the corrosivity classes it marks as recommended:

The first row matters to a designer: one coat of 80 um on bare steel is limited to class C2, which the scheme describes as unheated interiors where condensation may occur, and mostly rural exteriors, so a damp or outdoor steel piece wants a primer coat or galvanising. A heated, clean interior such as a hotel is class C1, below the table.

The scheme also says how thickness is accepted, which is worth borrowing. Qualisteelcoat measures dry film with a gauge to ISO 2808 and takes a number of readings that rises with the area: at least five for up to one square metre or running metre, ten for one to three, fifteen for three to ten. The mean must reach the required thickness and stay within any maximum the powder maker sets, no single reading may fall below 80 per cent of the requirement, and, where the maker sets no maximum, none may exceed three times the nominal figure. On blasted steel the readings are corrected for the surface profile.

Pre-treatment: where the sources say failures start

Qualicoat UK & Ireland says the vast majority of powder coat failures can be put down to poor or inconsistent pre-treatment, which gives the coating both a seal and a key. The routes differ by metal:

Two points follow for the specification. First, name the route, or ask the coater to name it in writing before pricing, because a cheaper line may stop at iron phosphate, which Qualisteelcoat limits to class C3. Second, ask which scheme licence and which approved powder and pre-treatment system the coater will use, and for the certificate. Qualisteelcoat's certificate is valid for one year and is tied to the systems named on it.

Edges, welds and design for coating

A powder film does not build evenly over a sharp edge. Qualicoat UK & Ireland says sharp corners will not allow the coating to build to the correct thickness, which shortens its life and leaves the edge open to damage, and that cut edges should be linished to a 0.5 mm radius and burrs removed. Qualicoat's corrosion guidance lists an edge radius under 0.5 mm and burrs, a damaged film, contact with other metals, dirt in inside corners and a lack of drainage among the things the pre-treatment and coating cannot put right. A powder maker adds that mechanical preparation of steel can remove welding residues and break sharp laser-cut edges before coating.

The coating also does not hide the metal. Qualisteelcoat's appearance test, made at 3 m for an interior and 5 m for an exterior, in diffuse daylight, disregards irregularities that are an image of the steel surface or the galvanising visible through the coating. A visible weld, or a rough laser edge, can still show through a powder film. If the face must read flat, say what preparation is expected on the drawing: see our weld finish sheet and metalwork brief checklist. Contact with other metals still needs isolating: see galvanic corrosion.

Colour, gloss and texture

Neither scheme chooses your colour. Qualisteelcoat says colour and gloss may be measured if the customer and the coater agree to it, so they belong in your specification, with a signed sample. Give the colour reference, the powder maker's product, the gloss in gloss units at 60 degrees to ISO 2813, and the texture, and see our RAL, BS 4800 and NCS sheet. Qualisteelcoat's gloss tolerances, which match those Qualicoat lists, are: low gloss 0 to 30 units plus or minus 5, semi gloss 31 to 70 plus or minus 7, high gloss 71 to 100 plus or minus 10. These are tolerances for the scheme's own test panels, not a promise for a three-dimensional part.

Metallic and matt finishes need care in use. Qualicoat advises that metallic coatings contain mica or metallic flakes, and that cleaning or repair by rubbing or polishing may cause discolouration and irreversible damage; for matt finishes it advises washing with mild detergent and avoiding rubbing, which could change the gloss. Its care sheet recommends cleaning at least once a year in a normal environment and every three months in marine, industrial and swimming-pool environments. See cleaning and maintaining metal finishes, and for thickness against other finishes plating and coating thickness.

Architectural powder coating to [QUALICOAT] specification, class [2], from an approved supplier [with the SEASIDE endorsement for sites within 5 km of the coast]. Coater to hold a current licence for [class and endorsement] and to supply the certificate before work starts. Colour [RAL [ ]] to be the [maker and product]; gloss [ ] GU plus or minus [ ] at 60 degrees (ISO 2813); texture [smooth]. Cut edges linished to [0.5] mm radius and burrs removed before coating.

Powder coating system [P-ST2-M, Qualisteelcoat 5.0.1 Annex 3] for corrosivity category [C3]: pre-treatment [mechanical, Sa 2 1/2], primer [60] um and polyester top coat [80] um, total [140] um. Dry film thickness measured to [ISO 2808] on the sampling plan in [ISO 19840]: mean not below [140] um, no reading below 80 per cent of it. Weld spatter and burrs removed and edges radiused to [ ] mm before coating. Coater to state its scheme licence and the approved system in writing.

Colour [RAL [ ]], gloss [ ] GU plus or minus [ ] at 60 degrees. Appearance judged at [3] m (interior) or [5] m (exterior) in diffuse daylight, unaided: no wrinkles, runs, sags, craters or blisters. Control sample [S-01] coated by the coater on [the same metal and route], signed [date]. Stainless steel, brass or copper: coater to propose the system (pre-treatment, primer, powder, thickness, cure, tests) in writing before pricing, and a coated sample to pass an adhesion test before approval.

Standards and sources

• Qualicoat, Introduction to the QUALICOAT Specifications, 26th Edition (valid 1 January 2026): volumes available on written request.
• Qualicoat UK & Ireland: Guidance Notes 2 and 7 (April 2025); Qualicoat TIS Nos 1 and 4
• Qualisteelcoat Technical Specifications 5.0.1 (valid 1 January 2026): Annex 3 and sections 2.2.2, 6.12 to 6.14.
• BS EN 12206-1:2021 (aluminium); BS EN 13438:2013 and BS EN 15773:2018 (galvanised steel): BSI listings read; texts not read.
• PCI, Architectural Specification Comparison (March 2021): pre-dates the 26th Edition.

Questions we are asked

Is Qualicoat the same as BS EN 12206?

No. Both are for aluminium. BS EN 12206-1:2021 is a standard for the coating; Qualicoat is a certification scheme with licensed coaters, approved powders and pre-treatments, and inspections, and it lists EN 12206-1 among its test standards. Qualicoat UK & Ireland says Qualicoat also requires filiform corrosion tests that EN 12206-1 does not. Neither covers steel.

How thick should powder coating be?

It depends on the metal and the system. In a March 2021 comparison, Qualicoat classes 1 and 2 ask for at least 60 um on aluminium and class 3 for 50 um. Qualisteelcoat's steel systems are 80 um in one coat, 140 um in two and 200 um in three. State the system and how thickness is measured, not a single figure.

Can mild steel be powder coated for outdoor use?

Within limits. Qualisteelcoat marks one coat of 80 um on bare steel as class C2 only, and two or three coats as C2 to C4. Hot-dip galvanised steel with 140 um in two coats is marked up to C5. Edges, welds and drainage matter as much as the coating.

What do matt, satin and gloss mean in numbers?

They are not fixed bands. Give a gloss figure in gloss units at 60 degrees to ISO 2813 with a tolerance. Qualisteelcoat and Qualicoat list plus or minus 5 for 0 to 30, plus or minus 7 for 31 to 70 and plus or minus 10 for 71 to 100.

Can stainless steel or brass be powder coated to a standard?

We found no standard or scheme for them. Ask the coater to propose the system in writing, including the pre-treatment and primer, and approve a coated sample before production.

How do I clean powder-coated metal?

Mild detergent in water and a soft cloth, rinsed and dried, with no steel wool, strong solvents or strong acid or alkaline cleaners. Qualicoat recommends cleaning at least yearly, and every three months in marine, industrial and pool environments. See our cleaning sheet.