Painting galvanised steel sheet -
surface preparation and coating selection
Galvanised steel sheet can be painted with liquid paints as well as powder coated, and the combination of zinc with an organic coating, known as a duplex system, protects steel far longer than either layer on its own. Research on duplex systems puts their service life at 1.5 to 2.3 times the sum of the lives of the zinc and the paint alone. Two conditions have to be met: a properly prepared surface and a paint designed for zinc substrates. The vast majority of flaking and peeling on galvanised steel comes from skipping one of these conditions, not from the quality of the sheet itself.
Why paint comes off galvanised steel
A fresh zinc coating is smooth and chemically active. Paint has little to key into mechanically, and some binders react with zinc. On top of that, sheet from a continuous hot-dip galvanising line almost always carries a surface treatment intended to protect it against corrosion during transport and storage, not to prepare it for painting.
Chemical passivation forms a thin conversion layer that limits the formation of white rust, but it also reduces the adhesion of many paints. Protective oil works in a similar way: it protects the surface, but it must be removed completely before painting. White rust, the corrosion products of zinc, forms a loose layer on which no coating will hold for long.
A separate issue is alkyd paints, the common oil-based paints. Their binder reacts with zinc through saponification, which leads to loss of adhesion and peeling after a few months, even on a well prepared substrate. For this reason only systems declared by the manufacturer as suitable for zinc substrates are used on galvanised steel.
Surface preparation
The scope of preparation depends on the surface condition of the sheet. EN 10346 provides for several surface treatments, marked with a letter at the end of the designation, for example DX51D+Z275-N-A-C, where C stands for passivation.
| Surface condition | Designation to EN 10346 | Preparation before painting |
|---|---|---|
| oiled | O | alkaline or solvent degreasing, rinsing, drying |
| passivated | C | removal or conversion of the passivation layer: zinc activating agent, conversion coating or gentle blast cleaning |
| passivated and oiled | CO | degreasing, then as for a passivated surface |
| phosphated | P | degreasing is usually sufficient, the most favourable condition for painting |
| untreated | U | degreasing; the sheet must be stored dry, as white rust forms on it quickly |
| with white rust | any | removal of the deposit with a plastic brush or sweep blasting, washing, and with heavy deposits an assessment of the remaining zinc thickness |
Sweep blasting is a brief, gentle cleaning of the surface with a fine non-metallic abrasive at low pressure. It works well on thicker batch galvanised coatings. On thin sheet with a Z275 coating, where there is around 20 µm of zinc per side, chemical preparation is the safer option because it does not reduce the thickness of the protective layer.
Paints for galvanised sheet
| System | Application | Notes |
|---|---|---|
| epoxy primer and polyurethane topcoat | external structures and components, environments with increased corrosivity | classic duplex system to EN ISO 12944-5, the polyurethane topcoat provides UV resistance |
| waterborne acrylic paints for zinc | internal components, roofing, enclosures | only products declared as suitable for zinc |
| reactive primers | thin layer improving adhesion of subsequent coats | used under further coats, not as stand-alone protection |
| alkyd (oil-based) paints | not applied directly on zinc | risk of binder saponification and coating peeling |
The thickness and number of coats are selected according to the corrosivity category of the environment, from C1 in heated interiors to C5 in industrial and coastal zones. EN ISO 12944-5 gives ready-made paint systems for galvanised substrates, with expected durability from low to very high.
Powder coating galvanised sheet
Powder coating galvanised steel is standard practice in the production of enclosures, racking, gates and facade elements. Chemical pretreatment usually includes degreasing, a conversion coating (zinc phosphating or more modern zirconium-based products) and rinsing with demineralised water. Requirements for industrial powder coating of hot-dip galvanised steel are set out in EN 15773.
The main risk in powder coating is outgassing: during curing, gases may be released from the zinc coating and leave craters and pinholes in the finish. This mainly concerns thick batch galvanised coatings. Sheet from a continuous line has a thin, compact coating and outgasses to a much smaller extent. Where the problem does occur, parts are pre-heated above the powder curing temperature or powders designed for galvanised substrates are used.
Painting after forming or pre-painted sheet
An alternative to an in-house paint shop is organic coated sheet from a coil coating line, in line with EN 10169. It pays off for large series in standard colours. Painting a finished part made from galvanised sheet, on the other hand, gives full freedom of colour, coating thickness and shape: paint applied after bending does not crack on the radii, and cut edges are protected together with the rest of the part. For plants that cut, bend and weld components before painting, this is usually the more favourable solution.
Selection criteria in practice
Service environment of the part. The corrosivity category determines the paint system and whether paint alone is sufficient or a full duplex system is needed.
Surface condition of the sheet. The type of treatment (C, O, CO, P, U) defines the scope and cost of preparation. When enquiring about material, it is worth stating straight away that it will be painted.
Appearance requirements. For visible parts, a minimised spangle coating with an improved surface quality works better, as described in the article on types of coatings on steel sheet.
Painting technology. A powder coating line with chemical pretreatment will handle passivation and oil in its standard cycle. Manual painting with liquid paint requires more careful preparation on site.
Available steel grades, coating types and coil, strip and sheet formats are presented in the HEN-STOL steel sheet offer, with separate pages for galvanized steel sheet, cold-rolled steel sheet and hot-rolled steel sheet.
See how we can help
HEN-STOL supplies galvanised steel in coils, strip and sheets to plants that paint their parts in-house. Send us the specification with a note that the material will be painted, and we will check the surface treatment of the available lots and advise on the most favourable option.
Frequently asked questions
Can galvanised steel sheet be painted?
Yes. Galvanised sheet is painted with liquid paints and powder coated, and combining zinc with paint (a duplex system) extends protection life 1.5 to 2.3 times compared with the sum of both layers. The conditions are removal of oil, passivation and any white rust, and a paint designed for zinc substrates.
What paint should be used on galvanised steel?
A paint declared by its manufacturer as suitable for zinc: outdoors most often an epoxy primer with a polyurethane topcoat, indoors waterborne acrylic paints for zinc. Alkyd (oil-based) paints are not applied directly, because their binder reacts with zinc and the coating loses adhesion.
Does galvanised steel need a primer?
In most systems, yes. A primer for zinc substrates provides adhesion and forms the first layer of a duplex system. The exceptions are single-coat acrylic paints for zinc and powder coating, where adhesion is provided by the conversion coating from chemical pretreatment.
How is galvanised sheet prepared for painting?
It depends on the surface treatment: oiled sheet is degreased, passivated sheet is activated with a zinc agent or given a conversion coating, and white rust deposits are removed mechanically and washed off. On thin sheet, chemical preparation is safer than blasting because it does not reduce the zinc thickness.
Can galvanised sheet be powder coated?
Yes, it is standard in the production of enclosures, racking, gates and facades. It requires chemical pretreatment (degreasing, conversion coating, rinsing). The risk of outgassing concerns mainly thick batch galvanised coatings, while sheet from a continuous line outgasses to a much smaller extent.
Should galvanised steel be left to weather before painting?
No. Leaving sheet outdoors so that the surface dulls produces an uneven layer of corrosion products and poorer adhesion than controlled chemical preparation. Galvanised sheet can be painted straight away after proper degreasing and surface activation.
Sources
- EN ISO 12944-4, Types of surface and surface preparation, 2017
- EN ISO 12944-5, Protective paint systems, 2019
- EN 15773, Industrial application of powder organic coatings to hot dip galvanized steel (duplex systems), 2018
- EN 10346, Continuously hot-dip coated steel flat products, 2015
- J.F.H. van Eijnsbergen, Duplex Systems: Hot-dip Galvanizing plus Painting, 1994