Overcome Painting Hibernation.
Brisk air, bare trees and frost warnings used to be indications that it was time to hang up the brushes and rollers and put your maintenance painting program on hold until spring. Today, with advancements in coatings technology, cold and damp conditions are no longer obstacles to the painting process. High-performance coatings are now available for application in high- and low-humidity conditions, and at temperatures as low as 5[degrees]F (-15[degrees]C).
Moisture-cured urethanes, cold-cured modified epoxies and mastics and inorganic and organic zinc primers all perform well when the mercury drops. Unlike conventional protective finishes, these coatings are able to cure efficiently and form strong bonds to substrates in frigid and damp conditions. While it is still most appropriate to schedule maintenance painting during favorable weather or in heated enclosures, sometimes painting is unavoidable. By specifying the right coating system and surface preparation method, it can be done year-round.
Specifications are the key to a successful painting project. During cold-weather applications it is crucial to define criteria for surface preparation, acceptable environmental parameters, appropriate application methods and the coating system to be used.
When selecting a cold-weather coating system, one of the most important factors to consider is the ease of application under prevailing conditions. In general, the viscosity of a coating will increase at lower temperatures, which can make application with standard equipment difficult. Thinning may be required, but may also retard the cure. The coating selected should exhibit an acceptable viscosity level at lower temperatures, permitting application without excessive thinning.
Curing schedules are also critical in cold-weather applications. At low temperatures, typical air-dry and thermoset coatings cure more slowly. Among the coatings developed for low temperature application, there are a wide range of curing schedules for given temperature levels. The right coating will possess a curing schedule that meets the logistical constraints of the project.
When specifying a multi-coat system, caution must be exercised to ensure that the system integrity is not compromised by poor intercoat adhesion. Surface blooming is a major cause of adhesion failure in cold-weather conditions. Where adverse cure conditions are expected, only coatings with low-blooming tendency should be used in multi-coat systems.
Surface tolerance is another vital requirement for coatings applied in frigid conditions. It is always best to avoid painting over substrates contaminated with oil, salt or water. However, in cold-weather conditions, the presence of moisture is often unavoidable. Moisture-cured urethanes, commonly employed for cold weather applications, actually displace surface moisture to form strong bonds with the substrate or existing coating system. Single-pack, long-oil alkyds and epoxy mastics also tolerate less-than-ideal surface conditions, making them well suited for cold-weather application.
Systems for cold-weather applications
Of course, the primary purpose for applying protective coatings is to combat corrosion and chemical attack. So in addition to performing well during cold-weather application, the selected coating must be able to offer long-term protection against destructive variables such as condensation, humidity, UV light, wet/dry cycling, temperature cycling and the presence of harsh chemicals and solvents.
Specifying the right system is a matter of reviewing the characteristics of coatings developed for use during adverse weather conditions. These systems include inorganic and organic zinc primers, modified epoxies and moisture-cured urethanes.
Inorganic and organic zinc primers have been used as base coats on structural steel, piping, tanks and other steel substrates for protection against corrosion. While they are often applied in the heat of summer, they are also appropriate for cold-weather applications.
Self-curing Type 1-C inorganic zinc coatings, which are based on ethyl silicate resins, react with atmospheric moisture, making them moisture-tolerant. They also cure at temperatures as low as 5[degrees]F, but work best when applied at temperatures above freezing.
Epoxies have long been recognized for exhibiting good film integrity, water-resistance and toughness. In the past, epoxies were not intended for cold-weather application, but this shortcoming was eliminated with the development of new epoxy resins, hardeners and accelerators, as well as modified epoxy formulations.
Since their introduction in the 1970s, moisture-curing polyurethanes have gained widespread acceptance. The coatings are, in reality, single-component products that offer two-component polyurethane performance, with the second component being moisture on the structure's surface or in the atmosphere. Urethanes are well suited for this type of formulation because all isocyanates react with any compound containing reactive hydrogen such as water or alcohol.
Proper preparation is extremely important in low-temperature applications. As the limits of acceptable application temperatures are approached, preparation of the surface becomes more critical. To avoid pitfalls, consult with the coating manufacturer to address specific climate conditions peculiar to the job. It is also important to fully understand surface limitations described in product data sheets.
Cure temperatures as indicated in product specification sheets assume constant temperatures. However, it is more likely that weather conditions will change during the curing process. Air temperature, substrate temperature, wind speed and weather forecasts will affect any decisions on the advisability of application. In addition, temperatures cited by the manufacturer for curing almost always refer to surface temperature, not ambient air temperatures. Generally, temperature changes in the substrate, particularly with metals, will be slower to change than the ambient temperature, because substrates retain cold in winter and heat in summer much longer than air.
When specifying coating applications in cold environments it is important to include not only the minimum temperature and humidity levels at which the application may proceed, but also the length of time at which the temperature and humidity must be maintained above the minimum for proper curing. Steel temperatures on cold days may never quite reach ambient levels, particularly in sheltered areas. The heat-sink effect on the exterior walls of tanks and other vessels containing fuel or water can be enormous in the winter. It is the dwell time at a required cure temperature that is important and, under practical conditions, it is vital that the coating remain above the critical cure temperature long enough to achieve the necessary degree of cure.
When the substrate temperature drops below 32[degrees]F, painters should watch for ice formations. Ice is hard to detect, particularly on porous substrates such as newly blasted steel and zinc-rich coated surfaces. It is considered an unstable interference material, however, and should be removed prior to application of the coating. Eventually, the ice will melt, and if it has been accidentally coated, new coatings may lose adhesion.
Another area that requires special attention in cold weather applications is product storage. Even if a coating can be applied at very low temperatures, it should still be kept warm prior to application.
In addition, for flow reasons, it is helpful to maintain the elevated temperature during the application. In general, as the temperature of a resinous material decreases, it becomes thicker and more difficult to spray. Insulating the spray pot or using an electric heating coil around the pot will make the coating easier to spray without using excessive amounts of thinner that can adversely affect the coating's ability to meet thickness requirements. When applying coatings with spray equipment, keep hoses as short as possible, and wrap them with rags or insulation.
Getting it down cold
Advancements in cold-weather coating technology allow facility engineers to extend their painting season. But, as with any coating application, good surface preparation and proper coating specification are necessary to obtain long-term protection.
This includes a solid understanding of the product's characteristics and limitations. Proper education of the workforce, proper coating mixing and storage, and the selection of the correct application equipment and coating system can help ensure these types of applications are a success.
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|Publication:||Industrial Maintenance & Plant Operation|
|Date:||Oct 1, 1999|
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