Painting Water Tank Exteriors In-Service

Technology Exchange Makes Draining Tanks Unnecessary

By Tony Ippoliti
Senior Corrosion Specification Specialist
Sherwin-Williams Industrial & Marine Coatings

Until a few years ago, water tanks were always drained before exterior surfaces were painted. Condensation, common on water tanks below the water line, made it necessary to drain tanks so that the moisture didn't create a situation that would lead to a coating failure. But a coating technology borrowed from the bridge and highway industry and pioneered for a new use by Sherwin-Williams Industrial & Marine Coatings is bringing changes to the water tank marketplace.

During the last four years, at least 15 water tanks have been successfully painted while in-service. This is a breakthrough because in some municipalities and industrial facilities, draining water tanks can cause severe system strain. Small systems may not have sufficient excess storage capacity, and "taking down" a tank just to repaint the exterior surface may be unacceptable. Not only that, but when both municipal and industrial water tanks are drained, fire protection may be compromised. At the very least, water systems and their customers can be inconvenienced, particularly during peak usage and in times of drought.

What makes this Technology Exchange possible is moisture-cured urethane (MCU) coating technology. MCU's are available in several primer, intermediate, and topcoat varieties allowing specific systems to be created for each water tank. Moisture-cured urethane coatings are somewhat surface tolerant, quick drying, applicator-friendly and capable of application under conditions that would cause other coatings to fail. They are single-component, heavy-duty coatings that require no co-reactant and are expected to last, mil for mil, as long or slightly longer than the equivalent "standard" zinc/epoxy/urethane tank coating systems.

The successful use of moisture-cured urethane coating systems throughout industry has resulted in the American Water Works Association (AWWA) formally adopting moisture-cured urethanes as a tank painting system in the AWWA D102-03 Standard, as Outside Coating System (OCS) No. 2. The specification does not suggest that tanks may be painted while in-service but it does recognize the benefits of using MCU's for new or repainted water storage tanks.