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Painting over galvanised surface

We are having a job where the specification demanded a galvanising deposition of 275 gm/m^2. The manufactures mill test certifies the deposition, but when tested in independent lab, the result says the deposition is 170 gm/m^2 i.e. 25 to 30 micron tk. Is there any way to make it up by painting over it?

Goutom Bandyopadhyay
- Doha, Qatar
2006

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2006

Making up the thickness of galvanizing by painting is a problem.
First, if your spec is for galv and you supply paint, you are not supplying to spec. Next is performance: Galv and paint perform differently.

If your customer has set a spec, its for a reason:
1. They need that spec (in which case supplying something else might be a problem)
2. They didn't know what to specify and chose that spec almost by default. (in which case they might be able to "make do" with your proposed alternative)

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

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2006

Goutom:
When I first read your question I passed it up too quickly. Is the product general work or is it sheet? Sheet could be about any thickness but I have never seen lower than about 50 microns on general work (e.g. galvanized after fabrication). Thus if this is general work then the independent lab must be wrong and got a thickness that is lower than reality.
Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA

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2006

It is conceivable to get 25-30 you thickness on material like 6 mm thick. There are techniques to get coating thicknesses down to this. We used to do a run of pipe for a customer who wanted not more than 40 u. Wall thickness 4.5 mm, diameter 50 mm, mild carbon steel pipe.

But thicknesses below 50 you are not common as said above, and an error could have been made.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

Unfortunately the lab made a mistake, the obtained coating was 340 micron but reported half of that. Problem was solved.

Goutom Bandyopadhyay
- Doha, Qatar
April 7, 2009