Coating systems for steel structures: zinc, powder, primer with enamel, duplex

Four ways to protect steel and how they differ

Hot-dip galvanising, powder coating, a liquid system of primer and enamel, and duplex protect steel in different ways, so their limits differ too. The choice starts not with a preferred coating but with three questions: where the structure will work, what its size and shape are, and whether colour is needed.

Hot-dip galvanising gives a metallic layer fused with the steel: under DSTU EN ISO 1461:2024 the minimum mean coating thickness is 45 to 85 µm depending on the steel thickness (more in the article on zinc coating thickness). A powder coating is a continuous polymer film baked in an oven. A liquid system consists of several layers that cure in air. Duplex combines zinc with powder or with a liquid system.

The table compares the methods in principle. For a specific item, size, shape, environment and project requirements decide.

What we carry out

The company POLYGONAL carries out hot-dip galvanising to DSTU EN ISO 1461:2024 and powder coating, and also applies, in the shop, liquid systems of primer and enamel to project requirements, including over zinc. We carry out abrasive blasting on site and, where the technical requirement of the project calls for it, before painting: abrasive blasting. Materials, the number of coats and thicknesses are set by the project. We apply liquid systems mainly by airless spraying.

Different parts of one structure may have different protection, and then the KMD drawings need a list of what is covered with what.

Atmospheric corrosivity categories

The atmosphere is divided into categories by how much metal a control specimen loses in the first year. ISO 12944-2 distinguishes C1 (very low), C2 (low), C3 (medium), C4 (high), C5 (very high) and CX (extreme). Its loss limits are the same as in ISO 9223, on which our estimate of zinc service life in the article on galvanising service life is built.

This is the loss of standard specimens in the first year, not the wear rate of a particular structure. The values correspond to ISO 9223.

Macroclimate and microclimate

The category is set by the macroclimate of the site, but the microclimate can raise it. A place where water does not dry, the zone under a downpipe, a closed cavity with condensation, an area under road-salt spray: in such details the structure works in a more aggressive category than the district as a whole. For what developed corrosion does to a section, see corrosion wear of members.

In category C1 protection is in principle not needed; if painting is done for looks only, a system for C2 is taken. For CX the standard gives no typical systems: they are defined separately for each site.

Durability classes: low, medium, high, very high

ISO 12944-1 names four durability classes of a protective system. Durability here is the expected time to the first major maintenance of the coating, not a guarantee period and not the time to the first rust. It is a planning value for the maintenance schedule.

The durability class is set by the project together with the corrosivity category. These are planning guides, not a manufacturer’s guarantee.

What actual durability depends on

The standard names eight factors: the type of system, the design of the item, the condition of the substrate before preparation, the grade and quality of preparation, the condition of joints, edges and welds before preparation, the quality of application, the conditions during application, and the exposure conditions. The material is only the first of them. The rest rests on the project and the contractor, and thicker paint does not make up for poor preparation.

For paint on zinc, durability is tied to the adhesion of the film to the zinc surface. If the film is damaged, the zinc beneath it keeps protecting the steel, so duplex does not «reset» at a scratch. How long zinc itself lasts in each category is shown in the article on coating service life, and how galvanising differs from electroplating and cold galvanising in the article on galvanising methods.

Preparing carbon steel for a liquid system

For steel of rust grades A–C under ISO 8501-1 the minimum cleaning grade under a zinc-rich primer is Sa 2½ with a medium surface profile (G) under ISO 8503-1, unless the technical data sheet of the material requires otherwise. Sa 2½ means abrasive blast cleaning after which only barely visible traces in the form of a shadow or streaks remain. The table of Sa grades is on the page about abrasive blasting.

Rust grade D (mill scale has flaked off and pits are visible) is singled out by the standard: because of the greater roughness the film thickness or the number of coats should be increased and the material manufacturer consulted.

Cleanliness matters as much as the Sa grade. There must be no soluble salts, dust or oil on the surface. Freshly cleaned steel starts to rust within hours, so priming follows cleaning immediately.

If the technical requirement of the project is abrasive blast cleaning, we carry it out before painting.

Welds, edges, cut edges

Paint pulls away from a sharp edge, and the film there is thinner than on a flat face. So sharp edges are ground off, and edges, welds and corners get an additional stripe coat by brush before the general coat. Edges after gas, laser or plasma cutting are dressed; weld spatter, undercut and slag are removed before painting. The preparation grades P1–P3 for welds and edges are described in a separate part of ISO 8501 (ISO 8501-3); the required grade is stated in the project.

Pre-fabrication primer

If the sections are blast-cleaned and covered with a thin fast-drying coat (usually 15–30 µm) before cutting and welding, that coat protects only during fabrication, transport and erection. Normally it is not part of the system and is removed if necessary; if it is left, it is treated as part of the system and tested with it. Not every pre-fabrication primer is compatible with the primer of the system; compatibility is confirmed by the material manufacturer. The KMD drawings state whether there is a pre-fabrication primer and which one, or require it to be removed.

What a liquid system consists of

A liquid system is a sequence of coats with different purposes. The primer bonds to the steel and holds the following coats, the intermediate coat adds thickness and a barrier, and the topcoat sets colour, gloss and light resistance. If the system has one coat, it serves as both primer and topcoat.

This is a list of types, not a recommendation of brands. The material is chosen to project requirements with the paint manufacturer’s confirmation.

Two-component materials

Epoxy and most polyurethane materials are supplied in two containers: base and hardener. After mixing, the mixture is usable for a limited time (pot life), which shortens as temperature rises. An expired mixture is not used. Ethyl silicate zinc primers cure with moisture from the air, so at low humidity they cure more slowly. The limits on humidity and on wet and dry film thickness are taken from the manufacturer’s sheet, otherwise bubbles and pinholes appear, and an over-thick film cracks.

What we apply

The company POLYGONAL applies liquid systems in the shop of primer and enamel to project requirements, including over zinc. The type of primer, the presence of an intermediate coat, the enamel colour from RAL, the number of coats and thicknesses are project decisions. The project records requirements for the system (binder, number of coats, thicknesses), and the commercial material is chosen to meet them with the paint manufacturer’s confirmation.

How many coats and what thickness

The standard sets thickness as the nominal dry film thickness (NDFT), for each coat and for the whole system. The higher the corrosivity category and the required durability, the more coats and the thicker the system. In the standard’s examples on carbon steel the thinnest system is one coat of zinc-rich primer at 60 µm, and the thickest for C5 with very high durability is 320 µm. For hot-dip galvanised steel the examples run from 80 to 240 µm. These are guides from the 2018 edition (the current edition is 2019); the mandatory minimums for each combination of category and durability are in Annex B of the standard.

A thin film protects little, but an over-thick one is bad too: individual values are recommended to stay at no more than three times the nominal thickness, and some materials have their own critical limit in the data sheet. An over-thick coat dries slowly from the inside, retains solvent and cracks.

The nominal thicknesses in the standard’s tables are based on airless spraying. Brush, roller or conventional spray gives a thinner coat, so more coats are needed for the same thickness.

Thickness is checked on the finished system and, if needed, separately on the primer. On rough steel the measuring method is set by ISO 19840, on a galvanised surface by ISO 2808, unless the parties agree otherwise.

Application conditions: temperature, humidity, intervals

Painting is possible only when the steel surface temperature is at least 3 °C above the dew point. With a smaller margin an invisible film of condensate forms on the metal, and the paint goes onto a wet base. The probability of condensation is assessed under ISO 8502-4. A massive structure is colder than the air in the morning, so the calculation uses the metal temperature, not the air temperature.

The other limits are set by the technical data sheet of the material: minimum temperature, upper humidity, pot life, drying time and overcoating interval. The interval has two limits. Apply the next coat too early and solvent stays under it; too late and the next coat bonds badly and the previous one has to be abraded.

Before each following coat the previous one must be clean and dry, free of dust, grease and white zinc salts. Contamination is washed off with clean fresh water, grease is removed, and defects in the coat are corrected before the next application.

Shop or site

The standard advises applying most coats in the shop, and preferably the whole system. In the shop it is easier to hold temperature and humidity, to control application, to repair damage, and output is higher. Shop limits: the size of the item and the risk of damaging the coating in transport and erection. If later coats are applied on site, the maximum overcoating interval may be exceeded and the last coat gets contaminated. The company POLYGONAL applies liquid systems in the shop only: we do not apply them on site. We do not repair coating damage after erection: it is done by whoever erects the structure.

So the KMD drawings state which coats are applied in the shop and which after erection. The options differ: the whole system in the shop with damage repaired after erection, or primer and intermediate coat in the shop and the topcoat over the whole surface on site. Repaired places always stay a little visible, so where appearance matters, the topcoat goes over the whole surface.

Galvanised steel under paint

Zinc is smooth and chemically active, so paint holds on it worse than on cleaned steel. The minimum the standard requires for a hot-dip galvanised surface is sweep blasting: the jet runs at an angle and only matts the zinc without removing the layer. Ordinary blasting removes too much zinc, so the request says «sweep blasting» and not «blasting to Sa 2½».

Part of the preparation is done by the galvanizer, so painting is mentioned before dipping. Passivation against white rust worsens adhesion. Runs, drips and «icicles» show through the film. White rust, oil and contamination are removed before painting. What to demand of the item and the plant is in the articles on design for galvanising and preparation for galvanising, and on white rust in how to store galvanised items.

Etch primers and chemical pre-treatment of zinc are used only with the approval of the manufacturer of the system material. In the standard’s examples for zinc the primer is epoxy, polyurethane or acrylic; there are no zinc-rich primers there.

Powder over zinc

For powder coating over hot-dip zinc there is the European standard EN 15773:2018 on duplex systems: it defines what the customer, the galvanising plant and the coater agree between them. In practice this means heating the item before application so that moisture and air leave the pores of the zinc (otherwise pinholes and blisters stay in the film), sweep blasting instead of ordinary blasting, and coating immediately after preparation. Our procedure is described on the page about powder coating, and what affects the work in the article on powder coating cost factors.

Powder or liquid system: which to choose

Powder and a liquid system are not interchangeable. Item size, heating, repair on site and the normative base differ between them.

How to choose

Powder is considered for items that fit the chamber, have open surfaces and withstand heating. Long poles of 8–12 m do not fit the chamber whole: the sections that fit are coated. A liquid system is needed when the item is larger than the chamber, assembled from parts that cannot be heated, when the coating is applied or topped up after erection, and when the project requires a system under ISO 12944 with a given durability.

Typical mistakes

• painting over rust and mill scale without cleaning to the grade in the project: the film sits on a layer that itself flakes off, and rust under it keeps growing;
• paint on a wet or cold surface: if the steel is less than 3 °C warmer than the dew point, there is a film of condensate on it and adhesion is lost;
• coats from different manufacturers without confirmed compatibility, including a pre-fabrication primer that does not hold the primer of the system;
• a missed overcoating interval or an over-thick coat that cracks;
• sharp edges, welds and corners without a stripe coat: paint pulls away exactly there, the film is thinner, and rust usually appears there first;
• gaps between plates, bolts, nuts and washers left unprotected. Corrosion develops faster in gaps; they are closed with a weld or sealant, and fasteners are galvanised or painted as part of the system (for the fastener class and galvanising see fastener strength classes);
• paint on the contact surfaces of a slip-resistant joint although the calculation assumed bare metal;
• an epoxy enamel as an outdoor topcoat where colour is needed: in sunlight it dulls and gets a chalky bloom;
• the galvanizer was not told about painting over zinc, and the item is passivated or has runs.

What to put on KMD drawings and in the request

Corrosion protection is written so that the contractor does not have to interpret: category, durability, system, preparation, places, inspection. All values in the table are set by the project; for the general rules of drawing up the drawings see KMD rules.

Slip-resistant joints and fasteners

The contact surfaces of a slip-resistant joint are not painted unless the project says otherwise: friction between plates depends on the condition of the metal, and a thick coating lowers it. ISO 12944-5 requires that in preloaded joints the system does not unacceptably reduce the preload; the treatment method and friction values are set by the calculation. How to state such surfaces is covered in the article on bolted connections.

Fasteners are written separately: galvanised, or painted as part of the system. The design resistances of high-strength bolts in DBN are given for bolts without a coating, so a change of fastener coating is agreed with the designer.

What to send for a quote

Send KM or KMD drawings, the project requirements for the system (category, durability, thicknesses), the surface condition, the colour from RAL; the zones without coating. Skip what you do not know: we will clarify during the calculation. We fabricate steel structures to the customer’s drawings, prepare KM and KMD drawings (KM and KMD design, steel fabrication), and carry out hot-dip galvanising, powder coating and liquid systems to project requirements. Erection is done by our partner construction company.

Standards and validity

Corrosivity categories and durability classes are set by the ISO 12944 series «Paints and varnishes. Corrosion protection of steel structures by protective paint systems»: part 1 (2017) general introduction and durability classes, part 2 (2017) classification of environments (confirmed in 2023, a new edition is being prepared), part 3 (2017) design considerations, part 4 (2017) types of surface and surface preparation, part 5 (2019) protective paint systems (marked for revision), part 7 (2017) execution and supervision of paint work, part 8 (2017) specifications for new work and maintenance (a new edition is also being prepared). The new editions are still in preparation; the ones named are in force.

Surface preparation and inspection are covered by ISO 8501-1:2007 (Sa grades; confirmed in 2017, a new edition is being prepared) and ISO 8502-4:2017 (probability of condensation); surface profile by ISO 8503-1, film thickness by ISO 2808 and ISO 19840, preparation grades of welds and edges by ISO 8501-3. For the last four the current edition is checked before citing.

Hot-dip galvanising of steel structures is covered by DSTU EN ISO 1461:2024, which we follow; powder coatings over hot-dip zinc by EN 15773:2018 (duplex systems). Design and execution of steel structures follow DBN V.2.6-198:2014 and DSTU EN 1090-2:2019. Which current DSTU reproduces ISO 12944, ISO 8501-1 and ISO 8502-4 could not be confirmed from open sources, so this is checked before citing in project documents.

The figures in this article are for orientation. The nominal thicknesses from the example systems are taken from ISO 12944-5 in the 2018 edition, which the 2019 edition replaced; a project uses the current text of the standard and the technical data sheet of the chosen material.