Powder coat vs wet paint on metalwork

At a glance

Corrosivity categories C1 to C5 and durability bands are those of ISO 12944; see our sheet on corrosion categories. 'Not evidenced' means we found no source for the statement, not that it is untrue. Figures for powder are from the sheet on powder coating metalwork; the paint figures are examples for structural steelwork in buildings, not decorative metalwork.

Four questions that decide it

Start from the piece and the place, not the finish. The drawing gives four prompts. None of them names a winner on its own: they tell you what to settle with the partner coater before a price.

1. Does the whole piece fit the coater's oven? A powder film is cured in an oven, so the oven sets the largest piece. We found no oven size for any coater: ask for the largest part size and weight, and for the pre-treatment line that comes with it. A piece too big can be coated in sections and assembled, which moves the problem to the joints; the other route is paint.
2. Will it be cut, welded, drilled or fixed after coating? Anything done to the piece after it is coated damages the film. steelconstruction.info lists recoatable finish coats for application on site, and says brushing and rolling are more common on site; Qualicoat's care guidance sends powder repairs through the coating supplier. Agree the repair method and who does it before approving either process.
3. Is it outside, damp or coastal? From category C3 upwards the coating is what protects the steel, so name the whole system, the corrosivity category and the durability you expect. For paint that is an ISO 12944-5 system; for powder, a Qualisteelcoat system or a galvanised-plus-powder system (duplex). A heated dry interior (C1) needs no corrosion coating; the choice there is about appearance and wear.
4. Are there sharp edges, welds or fine detail on show? Powder builds thinly on a sharp edge, so Qualicoat advises a 0.5 mm radius and removal of burrs. Wet paint has its own detailing guidance in ISO 12944-3, which we did not read. Either way the coating does not hide a rough weld: see weld finish and visible welds.

If the answers do not settle it, ask two coaters for a coated sample each, on the same metal and by the route they would use, and compare them in the real light. That is the one test that does not depend on a source.

What each process is, and what the sources say

Powder coating is a dry powder sprayed onto a prepared surface and cured in an oven into a film. The schemes that govern it are Qualicoat for aluminium and Qualisteelcoat for steel, set out in our powder coating sheet. Qualisteelcoat's steel systems are 80 µm in one coat, 140 µm in two and 200 µm in three; one coat on bare steel is limited to category C2.

Wet paint here means a system of liquid coats: a primer, one or more intermediate coats and a finish. steelconstruction.info describes primers as metallic-pigmented (zinc-rich, the most commonly used of that type) or binder-reliant (such as two-pack epoxy), and lists two-pack epoxies and urethanes among the generic paint types. The paint standard is BS EN ISO 12944-5:2019, which picks systems by category and durability. Protection, the page says, is directly proportional to dry film thickness.

Two statements are worth keeping in view. ISO 12944 durability is the expected life to first major maintenance, not a warranty, and the Qualisteelcoat scheme says the same of its own ratings. And Qualisteelcoat notes that ISO 12944 does not cover powder, so the two sets of figures are not interchangeable: do not write a paint durability band against a powder system.

Example paint systems for steel in buildings

steelconstruction.info publishes standard systems for structural steel in buildings, with the nearest ISO 12944-5 system, the preparation, and the coating life. They are for structural steelwork, not for fine decorative metal, but they show how a paint system is written: a preparation, then coats, each with a film thickness. Primer thicknesses may include a prefabrication primer.

Three points from the source. The film thickness that counts is the dry film, which is the wet film multiplied by the percentage of volume solids. Readings below 80 per cent of the nominal thickness are unacceptable and those between 80 and 100 per cent pass only if the mean reaches the nominal; Qualisteelcoat sets a similar 80 per cent rule for powder, so one acceptance wording can serve both. And micaceous iron oxide (MIO) intermediate coats have a limited colour range, so the colour usually comes from the finish coat.

Galvanised steel: paint or powder over zinc

Hot-dip galvanised steel can be painted or powder coated; the result is called a duplex system. steelconstruction.info lists a mordant wash, an etch primer or sweep blasting as ways of preparing a galvanised surface for paint, and Qualisteelcoat's galvanised systems are marked up to C5 with two coats totalling 140 µm. Powder over galvanising has its own agreements standard, BS EN 15773:2018, between client, galvaniser, chemical supplier and coater. The zinc, the preparation and the coating are one system, so ask the partner to name who is responsible for the whole of it. See powder coating metalwork for the powder side and galvanic corrosion where other metals touch it.

Coating system to BS EN ISO 12944-5 for corrosivity category [C3], durability [High]. Preparation: blast clean to [Sa 2½ to BS EN ISO 8501-1]. Coats: [zinc phosphate epoxy primer 80 µm; epoxy MIO 100 µm; recoatable polyurethane finish 60 µm]; nominal dry film thickness [240] µm. Colour [RAL [ ]], gloss [ ] GU at 60 degrees. Partner to state the system, the paint maker's data sheet and the applicator in writing before pricing.

Powder coating system [Qualisteelcoat 5.0.1, Annex 3] for category [C3]: [pre-treatment, primer and top coat in µm]; total [140] µm. Coater to state its scheme licence and the approved system in writing. See the sheet on powder coating metalwork for the full wording.

Dry film thickness to be measured on the plan in [ISO 19840]: mean not below nominal, no reading below 80 per cent of it. Control sample [S-01] on [the same metal and route] signed [date]. Repair of damage after coating to be by [the coater / the paint maker's recoat procedure], agreed in writing before work starts, with a trial on [the sample].

Standards and sources

• steelconstruction.info (SCI and BCSA): standard corrosion protection systems for buildings; protective coating specification; paint coatings: cites BS EN ISO 12944-2:2017 and -5:2019; read 7 Nov 2026.
• BS EN ISO 12944-5:2019 (paint systems); ISO 12944-1:2017 (durability bands): named from the sources above; texts not read. Confirm the current edition.
• Qualisteelcoat Technical Specifications 5.0.1 (valid 1 January 2026): as read for TL-MET-21.

Questions we are asked

Is powder coating better than wet paint?

Not in general. They suit different pieces: powder is a shop finish limited by oven size and repaired through the coater, wet paint can be applied or touched up on site and is specified to ISO 12944-5. Settle size, site work, exposure and edges first.

Can powder coating be touched up on site?

Qualicoat's care guidance says repair is through the coating supplier, and warns that rubbing or polishing metallic and matt finishes can discolour or change the gloss. Agree the repair method with the coater before you approve powder for a piece that will be altered on site.

Which finish for an exterior steel balustrade?

From category C3 up the coating protects the steel, so name the whole system: either a duplex system (galvanised plus powder) or an ISO 12944-5 paint system, with the category and durability. Rain-washed inland steel is usually C2 to C3; a coastal site can reach C5. See our sheet on corrosion categories.

Will wet paint match my powder colour?

We found no source that says. Give each coater the colour reference and ask for a coated sample on the same metal, then compare them together in the real light. Gloss should be given in gloss units at 60 degrees.

What is 2K polyurethane and MIO?

2K, or two-pack, means a base and a hardener mixed before use. Polyurethane is a common finish coat for outdoor resistance. MIO is micaceous iron oxide, a flaky pigment in an intermediate epoxy coat that builds film; because its colour range is limited the finish coat sets the colour.

How is thickness checked on each?

Both are measured as dry film. A common acceptance rule for each is that the mean reaches the nominal figure and no single reading falls below 80 per cent of it. Say how many readings and where.