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January 29 - February 2, 2018

How do you overcoat an aged inorganic zinc coating?

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From Robert Mexter of RMEX Consulting on January 30, 2018:
Based upon my past experience with inorganic zinc rich primers, I have an example of how we coated aged, inorganic zinc rich primers. When I say aged, the zinc-coated structural components were primed and left outside for two or three years ... not sure if that is " old enough" I will try to keep this short and simple for the readers. A bit of back ground: The project involved installing approximately 2.2 Million KG of large ( 1 inch thick) inorganic zinc coated structural steel and piping to reinforce (seismically) three, three-hundred-foot surge towers. Each tower has eight, 200-foot legs and on top of each tower was a forty-foot diameter x 96 in height tank. A total of 30,000 bolts were required and (drilled) per tower. Prior to site construction, all new structural reinforcement steel was abrasive-blast cleaned (automated blasting) in a controlled shop, with steel shot/ grit combination to meet SSPC-SP 10 near white metal. The resulting profiles was measured at 2.5 mils to 3.5 mils. The structural steel was then coated with 2.5 mils to 3.5 mils of inorganic zinc rich coating. I realize many of you will say "it barely covered the blast." However, with the chance of mud cracking and the issues with that, the best solution was to coat (minimum DFT) and cure. The sections of steel were then checked and re-checked, to ensure we had reached a proper cure (MEK rub test). Once we were satisfied that the sections had cured properly, we packaged and shipped to site. (This is where the aging comes in. As one can imagine, with this amount of steel on site, a lot of the steel was left to age on site during installation. It took approximately two years to install all zinc-coated sections. Once we had completed the installation work on each of the towers, and weather conditions were good, we began a three-year painting program. All aged inorganic zinc was power washed to removed  dirt or debris and any surface contaminants from installation. We chose to use a water-based acrylic primer and a water-based top coat. We completed this project twenty five years ago, and to date we have had no failures.Tthe three surge towers still look aesthetically excellent.

From Cristiano Godoy of KCC Safety on January 30, 2018:
Overcoating an aged IOZ requires skill and care. Once the coating has aged, porosity is no longer an issue. Also, as long as the surface is thoroughly clean and free from loose contamination, it will be suitable for overcoating.

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Tagged categories: Coating Application; Overcoating; Paint application; Zinc-rich (inorganic)

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