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Is powder coated galvanized steel a good exterior finish?

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I am detailing an exterior steel structure for a building and have labeled the exterior steel to be hot dipped galvanized and powder coated white. The intent of this note was to achieve the protective properties that galvanizing offers with the low maintenance of the powder coating. After further exploration of the web I found your home page and FAQ concerning the adhesion problems of the above. Is there another finishing process that can achieve the intent and/or is the powder coated galvanized steel finish the best solution for the exposed steel?

Raymundo Vela Jr
- Austin, Texas


Hot dipped galvanized steel uses the principle of galvanic corrosion as the protection of the mild steel. That is the zinc layer corrodes instead of the steel substrate. This will only happen with air and moisture from my understanding. Powder coat is an organic substance, as is most paints and is porous, thus allowing the moisture to travel through to the zinc layer. This can be deterred by using a chromate or zinc phosphate treatment on the zinc prior to powdercoating. As for adhesion , this can be achieved by either light sandblasting to provide a mechanical key, or by using acid etching for a chemical key. Powder should perform as well as any other paint, but the thicker the coat the longer it will take for corrosion to take place at the zinc layer. Perhaps using an epoxy based primer underneath the polyester powder top coat will help achieve a greater film build and subsequently a greater barrier. Your comment re low maintenance of powder may be slightly off center. Whilst it does not require constant repainting, it does require regular cleaning to remove oxides and maintain it's glossy appearance. Perhaps using a matte or low sheen finish may slow down the times it requires cleaning. You wouldn't leave your car unwashed for years without some form of maintenance would you?

Michael Lind
- Queensland, Australia


Thanks, Michael. Although I wouldn't leave my car unwashed for years, if I had towers for high-voltage wires I would leave them unwashed for decades; and if they were properly galvanized, they'd probably be okay despite the lack of attention. Sometimes maintenance of surfaces is do-able and sometimes not :-)

Raymundo might wish to further pursue this issue of paint adhesion on galvanizing via ASTM D6386 [link is to spec at TechStreet], "Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting".

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Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

+++++++

If hot dipped galvanized steel is "squelched", what does that mean?

John Gillen
Consulting Engineers - Mount Holly, New Jersey


Hi, John.

I've personally never heard that phrase and will take a wild guess that it is a mis-speak based on someone's faulty memory of the word "quenched". But if you can put it into a tighter context, there may be a better answer. Thanks.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

August 2, 2010

Hi. I'm a small powder coating company about to start coating a lot of galvanized steel. I've done little bits, but not very good ("pinholeing" or just not adhering) I don't want to lose the work so any help or advice would be appreciated

Graham Mcilhagga
powder coater - Sheffield, England


August 2, 2010

Hi, cousin Graham.
At the least, you'll need to phosphatize the galvanized steel before attempting to powder coat it. Are you pretreating to the spec that was mentioned, and still having adhesion issues, or you haven't had a chance to consult that spec yet? Have you been able to find that book about duplex coatings by any chance? If not, what process are you presently doing for pretreatment that is resulting in poor adhesion and pinholing? What kind of powder are you applying?

Good industrial quality coating is quite involved, and it truly isn't possible to condense it all down to a one-paragraph answer like "Ignore the books; just do this, and you'll be fine" :-)

But if you can tell us what you are doing, maybe we can spot a weak link. Thanks!

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


March 2, 2012

Dear Sir,

I would like to know if Pure Polyester powder is applied on MS, then how much temperature can it withstand in day to day use?

Regards,

Sanjay Mevada
- Ahmedabad, India


March 2, 2012

Hi. Sanjay.

Sorry, I don't know the answer myself. Please try your best to describe the actual exposure situation, and what motivates the question. We seem to find that it intimidates readers against attempting to answer if they feel they'll have have to think through and include a dozen "ifs, ands, & buts" that probably don't apply to your situation anyway :-)

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
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