Painting above-ground water tanks has its challenges. For one, the tank’s surface has to be dry. Condensation from the water in the tank makes this almost impossible, so standard painting practices call for draining the tank…which can really drain your budget.
The recent development of moisture-cured urethane coating technology has put an end to the requirement that above-ground water tanks have to be drained to paint. These single-component coatings are surface tolerant, quick drying, and applicator-friendly.
Moisture-Cured ChemistryA brief review of moisture-cured chemistry reveals what makes moisture-cured urethanes suitable for application to the damp surfaces of in-service water tank exteriors. The foundations of all urethane coatings are isocyanate groups. These groups react with any compound containing reactive hydrogen. With moisture-cured urethanes, that reactive hydrogen compound is provided by water vapor found in relative humidity.
|The 100-foot tall, single-pedestal, elevated spheroid water tank at the Rolls-Royce facility in Indianapolis was recently repainted while in-service. Using moisture-cured urethane coatings from Sherwin-Williams made draining the 500,000 gallon tank unnecessary.|
One of the most successful systems used to repaint water tanks in-service includes an MIO-aluminum-filled primer or intermediate coat. The micaceous iron oxide (MIO) and aluminum pigments in this coating align themselves more or less parallel to the substrate in closely packed layers. The overlapping pigment particles form a dense barrier against moisture and other forms of corrosive fallout that would attack the underlying surface. These pigments also act to reinforce the coating.
The MIO-aluminum-filled coating, when “power rolled” onto less than prepared substrates, provides “wetting out” properties and adhesion. It has proven to protect severely pitted areas on old tanks that have not been adequately maintained.
This unique chemistry does have its limitations. If there is too much moisture on the surface of the substrate, “outgassing” may occur resulting in pinholes in the coating film. But this phenomenon is easy to avoid: when the surface is wet with condensation, it is too wet to coat successfully.
If painting takes place and “outgassing” does occur, it is also easy to remediate: simply remove the damaged coating by scraping. Although moisture-cured urethanes can cure at temperatures as low as 20 degrees F, best results are achieved when water tank exteriors are painted at temperatures above 35 degrees F. At lower temperatures, the moisture levels in the air drop significantly, so the reaction rate is slowed. But the application parameters for these coatings are still quite large.
Case Study: Rolls RoyceMoisture-cured coatings make it unnecessary to drain water tanks for exterior painting. In the past six years, more than 20 water tanks have been successfully painted while in-service. The 100-ft. tall water tank at the Rolls-Royce facility in Indianapolis, IN is one of them. Built in 1942, the site was originally owned by General Motors and served as a weapons manufacturing facility for the war effort. In 1995, the plant was sold to Rolls-Royce. Although the name is synonymous with luxury cars, the company also produces gas turbine engines for commercial and military aircraft, and for the marine and energy industries.
|The moisture-cured urethane coating system from Sherwin-Williams proved to be the best choice for refurbishing the exterior of the Rolls-Royce water tank. In addition to providing a tough, long-lasting finish, the coating provided good looks as well.|
Water tanks like the single-pedestal, elevated spheroid water tank are common at industrial locations where immediate access to water is necessary in case of fire. The water tank at Rolls-Royce holds up to 500,000 gallons of water.
For a few reasons, repainting the water tank was necessary. The original GM logo needed to be removed and the old paint was showing its age with extensive fading, chalking and rusting due to coating erosion.
For plant managers at the Rolls-Royce plant, a moisture-cured coating system from Sherwin-Williams was determined to be the best choice for refurbishing the exterior of the company’s water tank. The coating system offered the corrosion protection required plus aesthetic value.
Surface preparation began with a Low Pressure Water Cleaning, SSPC-SP 12, WJ-4 (a maximum of 2500 psi was specified) to remove chalking followed by hand and power tool cleaning (SP2/SP3) to remove rust and loose coatings. Next, a moisture-cured urethane penetrating primer from Sherwin-Williams, Corothane I MCU Pre Prime was roller-applied at 1.5 to 2.0 mils dry film thickness (DFT). Following the application of the primer, rusted areas received a coat of Corothane I MIO-Aluminum at 2.0 to 4.0 mils DFT. For color and gloss retention, the final step was the application of the topcoat, Corothane I HS Aliphatic at 2.0 to 4.0 mils DFT.
The ability to achieve a specific color was especially important at the Rolls-Royce facility since the company uses a particular shade of blue in its highly recognizable logo. Sherwin-Williams is able to produce an exact color match since their moisture-cured urethane coatings are available in various tint bases. Without the tint bases, many colors cannot be obtained.
Using Moisture-Cured UrethanesMoisture-cured urethane coatings can be used to paint in-service water tanks at almost any time of the year except, of course, when there is the potential for dew or condensation to freeze on the tank surfaces. In this situation, containment and airflow may be used to prevent ice from forming. This holds true for plant piping systems as well.
Contractors using moisture-cured coating systems on water tanks like them because they are easy to use since they are single component and provide one-coat coverage. These advantages also allow the job to be completed quickly. For communities and industrial plants where water storage capacity is a concern, painting water tanks while in-service now is a proven, viable option.
Note from the author: The successful use of moisture-cured urethane coating systems throughout the 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 address tanks being painted while in-service but it does recognize the benefits of using moisture-cured urethanes for new or repainted water storage tanks. The National Fire Protection Association (NFPA) refers to this AWWA Standard (see NFPA 22 “Standard for Water Tanks for Private Fire Protection”, Chapter 12: Field Painting). The next revision to the NFPA 22 current edition (2003), will reference new coating systems adopted in the revised AWWA D102-03 Standard.