PL
ENPowder coating galvanised steel looks like a straightforward subject – after all, one protects against corrosion and the other improves appearance and durability. Yet this combination can cause plenty of trouble. The coating does not always want to hold, and the reasons often lie in details that are easy to overlook. Can galvanised steel be powder coated safely? It can – but you need to know how to go about it.
Zinc and powder – a problematic pairing
A zinc coating is there to protect steel against corrosion. It does this very well, but at the same time it makes coating more difficult. Zinc itself is a reactive metal – it oxidises on contact with air and water. It forms a passive layer that acts as a barrier to the powder. In physicochemical terms, it is a ‘difficult' surface.
On top of that, the structure of the zinc can differ dramatically depending on how it was applied:
- Hot-dip galvanising – produces a thick, rough layer with visible crystals (the so-called zinc spangle). An uneven surface and flux residues are a major challenge for the coating shop.
- Electrogalvanising (electroplated zinc) – a thinner, smoother layer, but mechanically less durable.
- Thermal (sprayed) zinc metallising – a different structure again, requiring its own approach.
Important
Not every galvanised steel can be powder coated in the same way. Even an identical product from two different suppliers can behave differently. That is why control matters right from the preparation stage.
Read also: Surface preparation for powder coating – stages, mistakes and best practice
What does correct preparation of galvanised steel for powder coating look like?
In theory it all seems simple: clean, coat, cure. In practice, galvanised parts call for a more rigorous approach than bare steel. The process usually looks like this:
Degreasing and cleaning
This is the first stage. It involves removing flux residues (especially after hot-dip galvanising), zinc oxidation products (oxides and carbonates), processing oils, dust, swarf, sweat and deposits. Sometimes you need dedicated degreasers for galvanised surfaces, with a pH chosen carefully to avoid micro-damage to the metallic layer.
Chemical activation or phosphating
This stage improves coating adhesion. The surface is gently modified chemically. In practice, the most common options are acid activation (e.g. phosphoric or citric acid) or zinc or iron phosphating. This layer improves the uniformity of application, reduces the risk of missed areas and increases the weather resistance of the final coating.
Rinsing and drying
After the chemical stage there is no room for haste. Every agent that could remain on the surface must be rinsed off thoroughly. Parts are dried at ambient temperature or with gentle heating. It is important that the surface is dry, clean and cool before it enters the spray booth.
Degassing (pre-bake)
This is the stage many people forget – and with hot-dip galvanising it is absolutely critical. The porous zinc layer traps gases and moisture. During curing, these gases escape and create craters, blisters and so-called pinholes in the coating. The solution? Pre-baking the parts in the oven before the powder is applied – typically 10–15 minutes at 200–220°C. The gases are released before the powder reaches the surface, and once cured the coating is smooth and uniform.
Which powder coatings should you use on galvanised steel?
The choice of powder matters a great deal – both for appearance and for durability. On the market you will most often come across two types that work well with galvanising:
Polyester powders
Resistant to UV radiation, weathering and moisture. Ideal for outdoor applications, including fencing, facades, signage and balustrades.
Hybrid powders (epoxy-polyester)
They combine the chemical resistance of epoxy with the better appearance of polyester. They work well in indoor and industrial applications, e.g. machine enclosures, electrical equipment and white goods.
TOPO powders – optimised parameters
TOPO offers powder coatings optimised for curing at 180°C part metal temperature for 10 minutes. This matters – a lower part temperature means less risk of damaging the zinc, a shorter cycle time and real energy savings in production. Remember that the oven temperature must be set higher than the target part temperature – the difference depends on the mass and geometry of the component.
With hot-dip galvanising, it is worth considering a two-coat system: a primer dedicated to galvanised surfaces + a topcoat. The primer improves adhesion and additionally blocks outgassing. In many cases this is the only solution that gives a stable, repeatable result.
Read also: Types of powder coating – differences, applications and selection
Curing parameters – no room for error here
The oven temperature and the time the part spends in the chamber must be matched to the powder, the wall thickness, the colour, the geometry and the properties of the material.
Too high a temperature can:
- ‘scorch' the powder (shade change, cracking),
- lead to chipping at the slightest stress,
- damage the zinc layer – and so expose the steel.
Too low a temperature will cause undercuring, giving a coating that is soft, short-lived and prone to scratching. Every powder has its own processing window – it is worth checking it in the technical data sheet or asking the manufacturer.
Read also: The role of the oven in the powder coating process
Applications of powder coating on galvanised steel
Powder coating galvanised steel is used in practically every industrial sector. It is worth considering if you work with:
- Fencing, balustrades, gates and shelters – where outdoor durability and appearance both count.
- Facades and cladding elements – which must meet aesthetic and environmental standards.
- Industrial machinery and equipment enclosures – where mechanical durability is as important as visual branding.
- Steel furniture and white goods – where practical function meets high hygiene requirements.
Common mistakes and how to avoid them
If you have problems with peeling, chipping or an uneven coating texture, there is probably a fault somewhere in the process. The most common causes are:
The most common causes of coating problems
- incomplete degreasing,
- no chemical activation,
- applying powder to a wet surface,
- an incorrectly chosen temperature and curing time,
- using an incompatible powder,
- skipping degassing with hot-dip galvanising.
Sometimes a single oversight is enough to leave a whole batch of parts fit only for blasting and reworking. The cost of rework can be severe – not only financially, but also in terms of deadlines and customer relationships. Better to catch it sooner rather than later.
How can you check whether the coating holds on galvanising?
Before you go into series production, run some tests. The simplest are:
- The cross-cut test – you score a grid of cuts, apply tape and pull it off. If the coating comes away, there is an adhesion problem.
- An impact or bend test – checks flexibility and mechanical resistance.
Specialist support
If you work with galvanised parts and are planning to powder coat them – don’t risk a trial-and-error approach; get in touch. We’ll advise you on the technical side, select the right system and help you match the powder to your production line.
Match the finish to your project
Choose a gloss level: matt, semi-matt, semi-gloss, gloss, high gloss.
Available surface textures: smooth, fine texture, coarse texture.
Need to check the coating before ordering?
Order a free sample.
