Until recently, water tanks always were drained before painting exterior surfaces. Condensation, common below the water line, made it necessary to drain to avoid a coating failure. However, a technology borrowed from the bridge and highway industry by Sherwin-Williams Industrial & Marine Coatings, Cleveland, is bringing changes to the marketplace.
During the past five years, at least 20 water tanks have been painted successfully while in service—a breakthrough because in some municipalities and industrial facilities, draining water tanks can cause severe system strain. Small systems may have insufficient excess storage. Also, fire protection may be compromised. At the very least, customers can be inconvenienced, particularly during peak usage and in times of drought.
What makes this possible is moisture-cured urethane (MCU) coating technology. MCUs are available in several primer, intermediate, and topcoat varieties, allowing specific systems to be created for each water tank. MCU coatings are surface tolerant, quick drying, applicator friendly, and can be applied under conditions that would cause other coatings to fail. The single-component, heavy-duty coatings require no co-reactant and are expected to last as long or longer than equivalent “standard” tank coating systems usings.
The successful use of moisture-cured urethane coating systems has moved the American Water Works Association (AWWA) to formally adopt MCUs as a tank-painting system in the newly revised 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 MCUs for new or repainted water storage tanks.MOISTURE-CURE PROPERTIES
MCUs are available with a wide range of physical properties and can provide color and gloss retention, and abrasion and impact resistance. In abrasion resistance tests, the coatings average only about 20 to 30 mg film loss, so windblown dust, sand, and soil do not cause significant damage. In impact resistance tests, several coatings have withstood 160 inch/pounds of direct impact.
The chemistry of MCUs does have one limitation. If there is too much moisture on the surface of the substrate, “outgassing” will occur, resulting in pinholes. But this phenomenon is easy to prevent: when the surface is wet with condensation, it is too wet to coat successfully. If outgassing does occur, it is easy to see and remediate: simply remove the damaged coating by scraping with a putty knife and move to an adjacent area of the tank that is not wet with condensation.
Although MCUs can cure at temperatures as low as 20° F, best results are achieved when water tank exteriors are painted at temperatures above 35° F. It has the proven ability to “extend” the water tank painting season in the cooler spring and fall months.SPECIFYING MOISTURE-CURE SYSTEMS
For color and gloss retention, aliphatic MCUs display better weathering characteristics; aromatics are used in primer and intermediate formulations.
Each tank painting project is unique due to the size, shape, and location of the tank; condition of the existing coating; and factors such as whether the tank will be overcoated or the existing coatings completely removed and the tank repainted. When the existing coating will be completely removed, a zinc-rich MCU primer provides the best corrosion protection. So that moisture is not incorporated into the primer, zinc-containing MCUs should not be agitated by the spray equipment during application.
Although surface-preparation methods may influence primer and intermediate selection, MCUs are somewhat forgiving of less-than-ideal surface preparation. Commercial blast cleaning to the Society for Protective Coatings standard SSPC-SP6, power-tool cleaning to SSPC-SP 2/3, and pressure washing to SSPC-SP12 (WJ1 and WJ4) all have been used successfully for in-service water tank painting projects. Flash rust-preventing solutions can be applied to WJ1-prepared steel surfaces to “hold” the prep until the primer is applied, and SSPC Visual Standards 3, 4, or 5 (as appropriate) can be used to verify surface preparation results.