In September 2022, I peeled a three-inch strip of anthracite grey powder coating off a galvanized steel stringer with my bare fingers. It came off like a sticker. Actually, like a decal—one clean sheet, no residue, no resistance.
I’ve been handling coating and fabrication orders at a metal shop in the Midwest for about eleven years, and I keep a documented list of my own significant mistakes. This one was somewhere around number nine. It involved powder coating over galvanized steel, a substrate I thought I understood. I didn’t.
The order that looked too easy
The project was a transit station canopy: 38 galvanized steel stringers, each specified with an RAL 7016 powder finish. The galvanizer had done its job. The parts arrived with a clean grey zinc coating, per ASTM A123. From the outside, fresh hot-dip galvanizing looks like a surface that’s ready to paint. The reality is more complicated: galvanizers often apply a passivation solution—sometimes chromate-based, sometimes clear—that protects the zinc in storage. That same invisible layer can stop powder from adhering.
We didn’t know that. We degreased, wiped each part down with toluene, masked, hung, and moved on. The first eight parts came out of the oven looking perfect. That made us confident. Then the ninth part came out covered in small craters, and over the next two days, more surfaces began to look like a lake after a hailstorm.
Our foreman asked if we should stop and investigate. I said, “Let’s adjust the powder and see.” That was a $5,800 mistake.
Where powder coating over galvanized steel goes wrong
Most buyers focus on the finish and miss the surface chemistry. The question everyone should ask isn’t “Will powder stick to zinc?” It’s “What is on that zinc layer before the powder hits it?”
Two things can ruin a galvanized job:
- Invisible passivation layers. A clean-looking galvanized part may be chemically engineered to resist moisture—and also to resist paint. Without a sweep blast or a conversion coating, powder has little to grab onto.
- Zinc outgassing. Fresh hot-dip galvanizing can release trapped moisture or alloy volatiles at typical powder cure temperatures. A pre-bake step at around 400°F for 10–20 minutes drives those gases out before the powder cures. We had no such step.
We had read the toluene SDS, so we knew we were working with a flammable solvent. What the toluene SDS couldn’t tell us was that toluene is not a surface conditioner. It removes oil, but it does not create a profile and does not neutralize a passivation layer. I treated it like a bond-enabler. It was actually just decoration.
The call to AkzoNobel Coatings Inc.
I called the technical service line at AkzoNobel Coatings Inc. and expected a series of product recommendations. Instead, the person asked me three questions:
- Was the galvanized steel fresh, or had it weathered?
- Did we pre-bake the parts before coating?
- What pretreatment did we use?
My answers were: fresh, no, and “a solvent wipe.”
His response was short, and I still think about it: “Powder coating over galvanized steel is about the surface, not the powder.” Then he explained the fix: sweep-blast the zinc to create a profile, pre-bake to stabilize it, apply a suitable primer, and topcoat. If the geometry allowed it, a factory-applied system—like AkzoNobel coil coatings—would remove those variables entirely by treating and coating the metal on a controlled line.
That last sentence changed how I spec future projects.
The fix and the real cost
We stripped the failures, lightly abrasive-blasted each part to avoid cutting through the zinc, added the pre-bake step, applied primer, and re-coated. The tape pulls per ASTM D3359 came up clean at 5A. We also ran a 1,000-hour salt spray coupon for peace of mind. The job shipped a week late.
Costs: stripping labor, new media, primer, extra electric for the oven, and one very uncomfortable call to the general contractor. Total, roughly $5,800 on a $31,000 contract. Not a storybook disaster, but real enough to remember. And yes—those first eight parts still make me nervous. Honestly, I’m not sure why they passed. My best guess is they came from a different galvanizing batch or had a slightly different surface condition.
When were acrylic paints developed—and why it still matters
During the re-work, a supplier asked me: “When were acrylic paints developed?” I gave the usual answer: the resin chemistry goes back to the early 20th century, and water-based acrylic house paints hit the market in the 1950s. But the deeper lesson is that the cure date doesn’t change the prep requirement.
Old acrylic and new polyester powder share one dependency: adhesion. If the substrate isn’t prepared, the binder doesn’t matter. This is also why coil coating is such a reasonable solution for galvanized components. The line controls cleaning, chemical treatment, primer, and cure in ways that a field-applied system cannot. When I compared a coil-coated sample with our booth-applied parts, I finally understood why process consistency is a feature, not a buzzword.
The checklist I maintain now
If you are powder coating over galvanized steel, here’s what I would check before the oven is even turned on:
- Confirm with the galvanizer whether the parts have a passivation treatment. Get it in writing.
- Pre-bake fresh hot-dip galvanized parts before powder if the zinc coating is substantial.
- Use a sweep blast or conversion coating to create tooth. Do not rely on a solvent wipe.
- If the part arrives as pre-coated coil, confirm the coating family. The AkzoNobel coil coatings documentation will tell you if a secondary bake is allowed.
- Read the SDS for every product you use—not just for safety, but to understand what it can and cannot do.
- Run a test coupon through the full process before production begins.
My experience is based on roughly 60 galvanized projects over eight years, mostly architectural and structural components. If you’re working with weathered galvanized metal, a different passivation chemistry, or a high-corrosion environment, your experience might differ. I can’t speak to every zinc alloy or galvanizer.
The way I see it, efficiency isn’t about moving faster. It’s about removing the hidden steps that slow everything down. For me, that means respecting the surface—and using AkzoNobel coil coatings where the job allows it. Take it from someone who peeled a perfect sheet of powder off a stringer: the surface was there all along. I just forgot to look at it.