In OCS No. 2, the AWWA recommends a minimum dry film thickness (DFT) of 7½ mils. Since each tank presents unique challenges and opportunities, other DFT amounts may be specified. Given all the variables, when specifying these systems, it is best to consult with an experienced and knowledgeable coating supplier offering a broad range of MCU coatings.WHEN TO USE MCUS
Moisture-cured coatings can be used to paint in-service water tanks at almost any time of the year, except in winter, 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.
For best results with MCUs, use only solvents recommended by the manufacturer. Occasionally, inexpensive “after-market” solvents may contain water, which will cause MCUs to become so viscous that workers are unable to apply them. When rolling MCUs, be careful not to dip water-dampened rollers into paint buckets, as the coating will begin to cure in the bucket; power rollers offer the best results. With all MCUs, once the bucket has been opened and the coating exposed to moisture-laden air, the curing process is initiated. As a result, all the coating must be either applied or, if unused at the end of the day, discarded following proper disposal procedures.
Contractors using moisture-cured coatings on water tanks say their projects move along remarkably quickly. For example, on the inaugural project—a 10-million-gallon water tank in Louisville, Ky.—the amount of condensation fluctuated, but no time was lost and the project was actually completed ahead of schedule. The temperature of the steel itself varied little from the 58° to 62° F temperature of the water contained within. Above the water line, relatively little condensation was present although surface temperatures occasionally soared to 120° F, demanding that painters schedule application for cooler hours during the hottest days. Once the micaceous iron oxide-aluminum coating was applied to roof and “knuckle” areas of this tank, however, surface temperatures dropped by an average of 20° F.
According to Mike Dudukovich, general manager of Kessenger Service Industries (KSI) LLC—the local painting contractor for the Louisville tank—moisture-cure coatings “opened up a window we've never been able to work through before. Using other systems we would have been getting about 35 hours a week under those conditions,” said Dudukovich. With MCUs, “we were able to work 70 hours a week.” KSI and other contractors have since painted tanks in-service, applying MCUs to standpipes, legged tanks, and other configurations in several states and on water tanks requiring various MCU combinations.
The moisture-cured system on the Louisville tank continues to provide superior adhesion and corrosion protection. The Louisville Water Co. and its coating supplier were both awarded the Red Ribbon at the Kentucky/Tennessee AWWA Conference and Exhibit in July 2004 for this first-of-its-kind undertaking. Several municipalities in Kentucky, Indiana, Ohio, and Minnesota have successfully painted at least one tank in-service. Some communities, satisfied with their experience, have subsequently painted more in-service tanks.
—Tony Ippoliti is senior corrosion specification specialist with Sherwin-Williams Industrial & Marine Coatings.
Using moisture-curing urethane technology, this water tank in Louisville, Ky., was painted without first having to be drained. Photos: Sherwin-Williams Industrial & Marine Coatings
Moisture-cured urethanes can be used on tanks that have been readied with a range of surface-preparation methods.