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"Powder coating over zinc spray surface"

An ongoing discussion beginning back in 2019 ...

January 27, 2010

Q. I am working on a project with a 2 meter tall chassis and will required a heavy duty anti corrosion coating. I have identified Zinc spray and which has passed all the required tests. However, the next problem that I have now is adding a cosmetic powder coating.

The Zinc sprayed surface is relatively rough affect the powder coating finishing. So far the only way to smooth out the surface by light sanding but this will not be able to guarantee the zinc coating coverage as sanding is not an easy and controllable process.

Is there any coating system will provide a high level of corrosion resistance and allowing a cosmetic powder coating. Giving the size of the part, it has been a unsuccessful finding a supplier locally to do zinc plating... by the way hot dip was not solution either as there are a lot of internal fasteners as well.

Gordon Wong
product designer - Ottawa, Ontario, Canada

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simultaneous January 28, 2010

A. It sounds like the spray paint is what we would call "zinc rich" and will contain 95% zinc. This is not designed for powder but as a 'stand alone' air-drying metal treatment.
You could use a 'zinc rich powder' (5-10% zinc) giving a smooth finish for a polyester top-coat... unfortunately, a two bake system.
Another alternative would be an air-drying waterborne primer specifically formulated for use under PC. I think a company from Toronto, Ontario, will be importing this for own use and resale State side.

Terry Hickling
Birmingham, United Kingdom

January 28, 2010

A. Better corrosion resistance and better adhesion achievable with hot dip galvanizing, and it presents a surface that's easily prepared for powdercoating.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
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January 31, 2010

A. I'm assuming that the coating you have identified is a metal spray coating, e.g. pure zinc, melted, atomised and sprayed onto your item - not a zinc rich primer. Metal spray is a great base coat for powder coating and does not need etch priming like hot dip galv. The rough surface provides a great key for the powder coat to take to.

Light sanding should be OK, just to knock the tops off but do make sure it is well vacuumed / air blasted before powder coating.

A common issue that you get with powder coating over metal spray is gassing (air expanding beneath the powder coat and creating small, popped bubbles in the surface of the powder coat). This normally happens if you have a poor quality metal spray coating or a thick zinc coating (say 100 microns +). There are various solutions to this but most common are pre-heating the substrate to a temp above the powder coat temperature, then powder coat and bake. Some people also use anti-gassing powders. Some also put on a thin powder coat first and green bake it (bake until not fully cured), then apply the final top coat of powder and bake).

This process of metal spray and powder coat is very common in Europe and is widely used on gates, fences, balconies, street furniture, children's playgrounds, bollards, security fences, etc.

Stuart Milton
- Dudley, West Midlands

February 15, 2019

Q. Interestingly, we're having a problem powder coating over an Aluminum Thermal Sprayed Coating. In the thread above, someone says to lightly sand down the piece to knock the tops off. We sand a bit more heavily, and are getting pinholes in our powder coat finish. Is it possible this is related?

Dennis Kelvey
Advanced Surface Finishing, Inc. - Westbury, New York, USA

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