Low-Build Inorganic Zinc and a Questionable Field Repair
A fabricator applied a specified inorganic zinc primer at 3–5 mil DFT to new steel. Nine months later, field inspection finds measured DFT of 2.0–2.6 mil over most of the fascia, white corrosion product on the top flanges, and mud-cracked areas near stiffener welds. The primer has been exposed to a marine atmosphere, and the fabricator's QC records show the primer was applied at 4.5 mil wet, below the volume-solids calculation for 3 mil dry. The field contractor proposes to spot-clean the white rust with hand tools, apply a surface-tolerant epoxy midcoat directly over the zinc, and continue with the polyurethane topcoat. The specification requires a mist coat over inorganic zinc and a 3-mil minimum primer DFT.
1. Evaluate the primer DFT nonconformance and decide whether the proposed repair is acceptable.
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The primer is nonconforming. At 4.5 mil wet and roughly 60% volume solids, the expected dry film is about 2.7 mil, consistent with the 2.0–2.6 mil measurements. The specified minimum is 3.0 mil. Low-build inorganic zinc reduces the amount of zinc per unit area and weakens galvanic protection; on a marine bridge this is a durability issue, not a cosmetic one.
Spot-applying a midcoat does not restore the missing primer. Options are to remove the deficient zinc and reapply, or to apply an additional compatible zinc coat if the manufacturer permits it and the resulting system can be validated. Given the broad area at 2.0 mil and the uneven film, full removal and reapplication is the defensible choice. Issue a nonconformance report, record the root cause, and do not let the topcoat proceed until the primer is accepted.
2. What is required to clean the white corrosion product and the mud-cracked areas?
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White rust is zinc corrosion product, not a surface that can simply be overcoated. Remove it to sound zinc using the coating manufacturer's written instructions, typically low-pressure water washing with bristle brushing, followed by drying. Avoid acid cleaners unless the manufacturer specifically approves them and residue testing is performed. Check chloride on the cleaned zinc.
Mud cracking near stiffener welds usually indicates local excessive film build, poor spray technique, or stress concentration. The cracked coating is discontinuous and must be removed to sound material, feathered, and rebuilt to the correct film. Hand tools may be adequate for small areas, but power tools are more reliable. After cleaning and repair, verify adhesion and film thickness before accepting the surface for the midcoat.
3. Was the contractor's plan to apply a surface-tolerant epoxy directly over the zinc appropriate, and what role does the mist coat play?
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No. The specification requires a mist coat, and inorganic zinc is porous. Applying a full epoxy coat directly can cause pinholes and outgassing as air escapes from the zinc layer, leaving holidays and weak spots. A thin mist coat wets out the porosity and allows air to escape before the full coat is applied.
A surface-tolerant epoxy is formulated to tolerate some surface contamination; it is not a substitute for cleaning white rust or for following the specified recoat sequence. The correct sequence is: clean and dry the zinc, apply a thin mist coat per the manufacturer, allow solvent flash, then apply the full midcoat within the recoat window. Verify with DFT and adhesion testing.