Recoating Window Missed on a Zinc-Rich Primer
A bridge girder is primed with an inorganic zinc-rich primer at 75 µm. The topcoat is a high-build epoxy specified at 100–150 µm. Weather delays push the topcoat 5 weeks past application of the primer. The primer surface now feels chalky and powdery, and a white residue rubs off on a gloved hand. The applicator proposes to spray the epoxy directly over the primer to avoid delay, arguing the epoxy will bond to the zinc. The specification requires the primer to be overcoated within the manufacturer's recoat window and requires removal of zinc salts before topcoating.
1. Explain why direct topcoating is risky here and what the white residue indicates.
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The white powder is zinc salt (zinc oxide/hydroxide or zinc carbonate) formed by reaction of the zinc with moisture and carbon dioxide in the air. It is a weak, powdery layer that prevents adhesion between the primer and the topcoat. The recoat window has been exceeded, so the primer has fully cured and its surface is no longer chemically receptive; direct application of epoxy over salt and chalk will likely produce intercoat adhesion failure, blistering, or delamination.
Spraying over it does not 'bond to the zinc' - it bonds to the salt layer, which is the weak link. The specification correctly requires salt removal before topcoating.
2. Describe the surface preparation and verification steps needed before applying the epoxy topcoat.
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Preparation: remove the zinc salts and chalk by fresh-water washing (and, where required, light abrasive sweep or high-pressure water) until the residue is gone; confirm with a salt test (e.g., conductivity or chloride test) and a visual check that the surface is uniformly grey with no white powder. Allow to dry fully and confirm the surface is within the recoat conditions and above dew point. If the manufacturer requires it, abrade or sweep the primer to restore a profile for mechanical adhesion.
Verification: document salt test results, visual cleanliness, surface temperature/dew point, and that the recoat window exception is handled per the manufacturer's written instructions. Then apply the epoxy within the specified DFT and inspect for adhesion and continuity. If the primer cannot be restored to an acceptable condition, the correct call is to remove and re-prime rather than topcoat over a compromised surface.