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Chalking of steelwork

Chalking of steelwork

(OP)
Hi All,

I have been contacted for technical advice on a coastal/beach development in Dubai (UAE, Middle East). The existing top coating of the external steelwork has been affected by the "chalking" phenomenon (see attached pictures).
The paint system has been specified by the Client as follows:
- Preparation, Grit bast clean to BS 7079,
- Primer. Zinc-rich Epoxy primer (60 microns DFT min)
- Build and finish coats. High build epoxy micaceous iron oxide (MIO) (min 200mirons DFT)
- Finish coat: Acrylic Polyurethane (60 microns DFT)

Do you have any experience with "chalking"? Causes and mitigation measures? Do you believe the specified paint specs mentioned above are suitable to an aggressive beach environment exposed to direct sunlight? if not, why?

Thanks

Regards

Simone

RE: Chalking of steelwork

Photos not attached.

RE: Chalking of steelwork

Chalking is almost always caused by degradation of the resin component of paint due to UV exposure. It could also be caused by poor application which resulted in the paint film not coalescing properly. An example would be applying a water based paint at a time of high humidity. Epoxy paint is typically poor in UV resistance, but urethane top coats with UV absorbers should be good.

RE: Chalking of steelwork

The specified system is a typical selection for a medium to high durability in a corrosive marine environment, Category C5-M. Refer to ISO 12944-5. The polyurethane should be aliphatic polyisocyanate cured for good resistance to chalking. When the paint manufacturers make a proposal get them to support their offers with test data including chalking resistance.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

RE: Chalking of steelwork

Hi Simone,

Steve is correct. I would recommend an acrylic aliphatic polyisocyante. Acrylics have excellent UV & Gloss retention. 2 urethanes our facility uses is Rustoleum 9800 & Gulf Coast Paint CT-398. Both products have over 3500 hours in the QUV which shows the weathering effect on coatings.

Thanks,

Andy

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