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March 20 - March 26, 2016

On guardrails on a marine wharf, grating panels consisting of mild steel bars about 1/8-inch by 1-inch on a 2-inch-square grid were painted with a three-coat zinc/epoxy/urethane system. Inspectors are finding numerous pinholes in the finish coat along the inside corners of the grating where the 1-inch-thick bars meet at a 90-degree angle. These pinholes were not noticed in the zinc or epoxy coats, but the final coat's visual inspection is more detailed. What is the likely cause of the pinholing, and what could be done to prevent it on future applications? Also, what should the disposition of the parts with pinholes be, as in, repair, accept as is, reblast and recoat, or an alternative?

Selected Answers

I think it is the high dry film thickness of the epoxy and the case of solvent entrapment where overcoating windows are also not adhered to before applying urethane topcoats.

From Francesco Colica of Colimet srl on April 7, 2016:
I agree with B Brown, i.e., the more zinc there is in the coating, the better the corrosion resistance, especially on guardrails on a marine wharf. Better than hot dip coating ( with normally 70 um Zn layer) is zinc thermal-spray process, whichproduces more than 250 um of zinc.

From B Brown of Self on March 25, 2016:
The details provided may be as lacking as the design. Are the overlaps simply overlapped, resistance welds, spot fillet weld or seal weld? Is the coating organic or inorganic zinc? How were the coatings applied - brush, roller, spray? If the pinholes weren't noticed until the urethane finish was applied, I bet you can scrape paint away to reveal a substrate that was not properly cleaned with mil scale present, particularly along the bottom edges of the overlap and if cleaned without decking on the side opposing the permanent decking on the edges of the overlap opposing the accessible side. You might want to check. I'll guess resistance spot welds with an open crevice all around each overlap and the overlap provides a surface suitable for pinholes to form. For such an intricate pattern hot dip galvanize would have been a good option versus a zinc prime coat. Maybe specify that the galvanize should not be lacquer- dipped and parts properly stored until installation. Otherwise, you will need a solvent clean before etching and epoxy urethane topcoat. If you go with a zinc coating, choose an organic zinc or maybe a water borne inorganic zinc. Have your inspector check the surface prep and prime coat WITH A MIRROR. The OZ or WB IOZ do allow for touchup. Choose an intermediate with a primer grade epoxy. Require brush stripe coat with the epoxy and then verify that crevices and pinholes have been sealed.

From James Prevatt of SPEC-GUARD on March 25, 2016:
You have specifically stated pinholes and not cracking of zinc is the problem. Cracking can occur up to several days after the application of the inorganic zinc. When using highly loaded zinc materials, a mist coat is critical to reach all surfaces to seal the zinc and not allow solvent to seep into the zinc and release after the next coat is curing or worse yet after it becomes mostly cured, creating pinholes. The likelihood of any coat being thicker than stated in the technical data sheet is great due to the configuration of the grate. Coating the grate with 90 degree angles, compounded by deep cavities is a matter of not having reasonable access to assure uniform thickness of each coat. This item lends itself to hot dipped application assuring all surfaces are coated and protected with a material which is not likely to crack even if applied with uneven thickness, in my view.

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Tagged categories: Asia Pacific; EMEA (Europe, Middle East and Africa); Epoxy; Inspection; Latin America; Marine Coatings; North America; Paint defects; Steel; Urethane; Zinc

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