August 12, 2009

Long-Lasting Exterior Tank Coating Systems

by Tony Ippoliti

Got a new tank project upcoming? Want the tank to last a long time? Want it to last a long time and look good, too? You’re able to do this if you’re aware of the AWWA D102 Standard and the Outside Coating Systems within it.

Outside Coating Systems (OCS) are defined in Section 4.3.1 as “coating systems for the exterior, weather-exposed surfaces of steel tanks” and refer to a “general type” of coatings but do not “limit acceptable materials to an exact formulation.” Different manufacturers may therefore use the same resin type – polyurethanes, for example – but are not required to have identical formulas.  This allows for resin, pigment, and solvent improvements to be formulated into coatings intended for the exterior tank surfaces. Formulators are allowed enhancements, specifiers are allowed choice, and tank owners get a competitive price from multiple suppliers. Everyone wins.

To that end, it’s known and accepted that polyurethane coatings, for the exterior surfaces of water tanks, are long lasting. These coatings, also used on steel bridges, ocean-going vessels, aircraft and automobiles, have been tested rigorously and found to be more than adequate to protect water tank exteriors. Polyurethanes resist deterioration by ultraviolet light and maintain their color and gloss for years. At least one polyurethane is available with a mildewcide to limit the defacement caused by such growth. It may also eliminate the tank owner’s maintenance expense of power-washing the tank every six to eight years.

In the 2006 revision of the AWWA D102 Standard, a fluorourethane topcoat – as part of OCS No. 4 – was approved as a finish coat that provides extended protection: “This system provides gloss and color retention exceeding that of aliphatic polyurethane finish coats.” (Section A.2.4). These finishes, used for decades on coil-coated standing seam roof structures, provide extended durability and colorfastness because the fluorocarbon resins they contain have low surface tension (adds weathering ability) and great cohesive bonds (adds resistance to UV). Some are graffiti-resistant.

New Joliet, IL tank

New Joliet, IL tank

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