Elevated Dew Point During Tank Coating
You are the coating inspector on a field-erected carbon steel storage tank. The interior is being lined with a solvent-based epoxy. The specification requires a minimum surface temperature of 50°F (10°C) and at least 5°F (3°C) above the dew point. The contractor has prepared the surface to SSPC-SP 10 and applied the first coat. The ambient temperature is 65°F (18°C), and the steel temperature is 55°F (13°C). The dew point is 52°F (11°C). The contractor wants to apply the second coat immediately because the first coat has reached a 'dry to touch' condition after 4 hours. The product data sheet states a minimum recoat interval of 6 hours at 70°F (21°C) and a maximum recoat interval of 7 days.
1. Should you permit the application of the second coat at this time? Provide a detailed justification using the given conditions and standard coating principles.
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No, the second coat should not be applied at this time. The primary reason is that the steel temperature (55°F) is only 3°F above the dew point (52°F), which is below the specified minimum of 5°F. This creates a high risk of condensation on the surface, which would be trapped under the second coat and lead to premature failure (e.g., blistering, poor adhesion). Additionally, the ambient temperature is 65°F, which is below the 70°F reference temperature for the recoat interval. The recoat interval must be adjusted for temperature; at lower temperatures, the curing is slower, so the 6-hour minimum recoat interval at 70°F would be longer at 65°F. The first coat may not be sufficiently cured to accept a second coat, risking solvent entrapment or intercoat adhesion failure. The contractor's observation of 'dry to touch' is not a reliable indicator of adequate cure for recoating. Therefore, the inspector should reject the application until the steel temperature is at least 57°F (to be 5°F above dew point) and the recoat interval has been extended according to the lower temperature, or until conditions improve.
2. What actions should you take to ensure the coating system's integrity before allowing the second coat?
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- Measure and record the steel temperature, ambient temperature, and dew point using a calibrated surface thermometer and sling psychrometer or electronic dew point meter. Ensure the steel temperature is at least 5°F above the dew point. 2. Verify the recoat interval: since the ambient temperature is 65°F, consult the product data sheet for the recoat interval at that temperature. If not provided, use the rule that cure time doubles for every 18°F (10°C) drop below the standard temperature. At 65°F (a 5°F drop), the cure time may be extended by approximately 20-30%. So the minimum recoat time might be around 8 hours. 3. Check the first coat for adequate cure: perform a solvent rub test or use a cure tester to ensure the coating is sufficiently cured to accept the next coat without solvent entrapment. 4. Ensure the surface is clean and free of contamination (e.g., amine blush for epoxy coatings) by washing with a suitable cleaner if required by the product data sheet. 5. Document all readings and actions. Only when conditions meet the specification and the coating is properly cured should the second coat be applied.