Yes, you can powder coat hot-dip galvanised steel, and it makes one of the longest-lasting finishes for outdoor steel. The catch is outgassing: air and moisture trapped in the zinc escape in the curing oven and leave pinholes and blisters in the powder.
Done properly, a galvanised-plus-powder system (usually called a duplex system) gives you the zinc's protection underneath and a coloured, wear-resistant skin on top. Done carelessly, it peels or looks like the surface of the moon. Here's why galvanising and powder don't automatically get along, what good preparation looks like, and what to sort out before your steel even goes to the galvaniser.
Why bother with a duplex system?
Each layer covers the other's weak spot.
Powder coating on its own is a barrier. While it's intact, the steel underneath is fine. Once it's chipped or scratched through, moisture reaches bare steel and rust can creep under the film from the damage.
Galvanising on its own protects steel sacrificially: the zinc corrodes first, even at small scratches. But it's dull grey, it develops white corrosion products, and in salty air the zinc gets used up faster.
Put them together and the powder protects the zinc, while the zinc protects the steel wherever the powder is damaged. That's why duplex systems are popular for gates, palisade fencing, balustrades, light poles, and coastal structures. If you're working near the sea, our guide to the best metal for coastal areas puts this in context with the other options.
What causes outgassing?
Hot-dip galvanising isn't a smooth, sealed skin at a microscopic level. The American Galvanizers Association describes two sources of trouble:
- Zinc oxides and other compounds on the surface can hold air and moisture.
- Tiny crevices in the outer zinc layer can trap air or water.
When the part heats up in the curing oven, that trapped air and water expand and push out through the melting powder. The result is pinholes, craters, or blisters. They're not just ugly: each one is a weak spot in the coating.
Galvanised steel also has surface features that cause problems: zinc dross particles (hard little lumps), zinc skimmings stuck to the surface, and drips and runs at the edges. Those need to be removed before coating, or they'll poke through the powder.
How galvanised steel is prepared for powder coating
The standard industry approach comes from the American Galvanizers Association and the ASTM D7803 practice for preparing galvanised steel for powder coating. In outline:
- Smooth the surface. File or grind off drips, runs, and dross lumps so the surface is even.
- Clean it. Remove oil, dirt, and zinc corrosion products. Chemical cleaning designed for zinc surfaces is common.
- Profile it lightly. A sweep blast (a light, angled blast with a soft, iron-free abrasive) roughens the surface for adhesion and opens up trapped pockets, without stripping the zinc. A full blast would remove much of the galvanising.
- Pre-bake it. Heat the part above the powder's curing temperature, typically for about an hour, to drive out trapped air and moisture before any powder goes on.
- Coat soon after. Apply the powder once the part has cooled enough, before fresh zinc oxide has time to form.
- Cure gently. Lower oven temperatures with longer cure times reduce outgassing compared with a hot, fast cure.
Powder choice helps too. Many manufacturers make outgassing-forgiving or degassing powders for zinc and cast surfaces. Ask the coater whether they use one.
The AGA's article on powder coating adhesion and outgassing goes into more detail if you're writing a specification.
What to tell your galvaniser
This is the step most people skip, and it happens before the coater ever sees the part.
Many galvanisers cool parts in a water quench or a passivation bath straight after galvanising. Passivation stops the zinc forming white storage stain, which is useful for bare galvanised steel, but it interferes with powder coating adhesion and can lead to blistering and peeling.
So when you send steel for galvanising, tell them it will be powder coated. Ask them not to passivate it, and to keep it as clean as possible. Then get it to the coater reasonably quickly: freshly galvanised steel has fewer zinc compounds on its surface than steel that has weathered for weeks.
Pre-galvanised sheet vs hot-dip galvanised parts
"Galvanised" covers more than one product, and they don't behave the same.
| Hot-dip galvanised after fabrication | Pre-galvanised sheet (galvanised at the mill) | |
|---|---|---|
| How it's made | Finished part dipped in molten zinc | Coil coated with zinc before it's cut and formed |
| Zinc coverage | All surfaces, edges, and welds | Faces only; cut edges and welds are bare or damaged |
| Coating thickness | Thicker | Thinner |
| Laser-cut edges | Cut before galvanising, so edges are zinc-coated | Cut after galvanising, so edges are bare steel |
| Typical parts | Gates, frames, structural steel | Enclosures, ducting, cladding, light brackets |
If you're having parts laser cut from pre-galvanised sheet, the cut edges lose their zinc protection, and welding burns it off around the joint. Powder coating covers those areas, but the edges won't have the sacrificial backup that a hot-dip galvanised part does. For harsh environments, that's a reason to hot-dip galvanise after fabrication, or choose a different material.
Design and planning tips
- Finish all fabrication before galvanising. Cutting, drilling, and welding after galvanising leaves bare steel and spoils the duplex system.
- Include vent and drain holes in hollow sections. The galvaniser will specify where they need them; they also help the coater.
- Round sharp edges where you can. Powder pulls thin on sharp edges as it cures.
- Keep fit-critical holes and threads in mind. Galvanising adds thickness, and powder adds more on top. Mask threads or chase them afterwards. Our guide to masking for powder coating explains how to mark them.
- Pick a polyester powder for outdoor use; epoxy chalks in sunlight.
If you're still deciding whether powder is the right topcoat at all, powder coating vs painting covers when wet paint is the better option, and what is powder coating explains the basic process.
Getting galvanised parts coated
We coat parts we make and parts customers send in. If yours are galvanised, the quote is where we sort out preparation with you, so tell us:
- That the parts are galvanised, and whether they're hot-dip or pre-galvanised sheet
- Roughly when they were galvanised, and whether they were passivated if you know
- Colour by RAL code, name, or photo, plus gloss level
- Any threads, holes, or faces to mask
- Where the parts will be installed
We confirm the colour with you before coating. See our powder coating service, or start a powder coating quote.