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Beware Potential Roof Coating Application Problems

By Kelly Franklin - June 2014 - Roofing


It is essential to apply products properly to ensure a successful coatings application. Application thickness, rainfall and cold temperatures all can harm roof coating longevity. Smith says managers should be aware of these potential problems:

Coating too thinly. Single-component systems, whether water- or solvent-based, lose volume as their films form because a portion of their composition is taken up by molecules of water or solvent that evaporate or flash off. Consider a coating of 70 percent solids by volume that is intended to dry to 22 mils. Due to the 30 percent water or solvent volume reduction upon application, the coating must be applied at 32 wet mils to dry properly.

If the same 70 percent solids coating is thinned on site and diluted, a 32 wet mils application would have a higher percentage of evaporated materials and would not yield the proper dry mil thickness. Coatings are formulated to wet out properly at specific wet thicknesses and to have suitable strength once dried. If the first condition is not met, the second one will likewise not be met.

Coating too thickly. In moisture-cure systems, applying a coating too thickly can lead to blisters or bubble entrapment because the carbon dioxide formed during the chemical reaction process cannot leave the film fast enough. When applied too thickly, water-based coatings can wrinkle when the film begins to form over the remainder of the wet coating drying beneath, not allowing water and slower-evaporating solvents sufficient time to pass through.

Mixing improperly. In some coating formulations, raw materials vary greatly in density, which might mean the heavier components pass through the solvents and plasticizers and settle at the bottom during shipment. Upon opening the container, it might not always be apparent these solids have settled. It is always good practice to mix the components before application, without mixing so vigorously that air is introduced into the coating, which can create surface bubbles later. Drill-powered paddles and 1- or 5-gallon stir sticks are useful tools for proper mixing.

Applying before rainstorms. Many coating manufacturers recommend a minimum of 7-12 hours after coating before an anticipated rain event, and for good reason. Rain on an undried coating can cause major surface defects and even wash off the coating. In moisture-cure systems, like polyurethane, water droplets on an uncured surface can cause surface defects or excess bubbling, due to the moisture cure reaction releasing higher amounts of carbon dioxide, or they can stop the reaction from continuing.

Cold-weather misapplication. Temperature affects the curing speed of single- and multiple-component formulations. A moisture-cured coating will cure faster in hot, dry conditions than it will in cool conditions. Likewise, a water-based elastomeric coating will dry faster on a warmer surface. In colder weather, the film forms more slowly, meaning that it might not have enough strength to hold up to a premature rain shower. Picone says frost also can contaminate the coating and impede proper adhesion if the coating undergoes a freeze/thaw cycle before curing.

The good news is that many manufacturers have trained certified contractors who, in accordance with manufacturer instructions, watch the weather forecasts and plan installations around proper temperatures and conditions. They also measure coats with wet mil gauges to be sure they apply materials at proper thicknesses.

Avoiding these five pitfalls can ensure that managers maximize the investment in roof coatings.

Spotlight: RCMA

The Roof Coatings Manufacturers Association (RCMA) is the national trade association representing roof coating manufacturers and suppliers to the industry. Founded in 1982, the association provides a unified voice to advance, promote, and expand the national and international market for roof coatings through education, technical advancement, and advocacy of industry issues.

For more information on proper roof coating selection and application practices, and to find a roof coating manufacturer, please visit the RCMA website at www.roofcoatings.org or call (202) 591-2452.





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