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Paints for Top-Coating Clearco Zinc Rich Primers
Zinc-rich paints are an accepted method of repairing damaged galvanized coatings according to ASTM A 780. Zinc-rich paints containing at least 65 percent meet the specification designations. They are widely used for touch-up and repair of damaged galvanized coatings because of their relative ease ofapplication. Although zinc-rich paints are useful as primers to gain surface adherence, they are also satisfactory as a finish coat when a neutral, or matching, or gray color is desired. These paints can be used alone, but for a more attractive finish, a topcoat is often employed. While most topcoats are easily used, some with very strong solvents may result in a lifting of the primer. Successful topcoatsinclude polyvinyls, acrylic latexes, polyurethanes, and polyamid cured epoxies. Specific manufacturer’s recommendations should be followed for application and top coating. Acrylic: -Acrylics are single-component coatings, generally applied over a primer due to thin film build. A wash primer may be used with these paints, or they may be applied directly over the hot dip galvanized surface. If thepH of the paint is high, problems may occur due to ammonia reacting with the zinc. Acrylics provide exceptional gloss and color, combined with an extremely durable finish. Aliphatic Polyurethanes: - This is a two-component, high performance system generally applied over a polyamide epoxy primer or a wash primer. These polyurethanes have superior weathering and chemical resistance characteristicswith good adhesion, as well as an enamel-like finish. This system requires strict attention to application procedures. If top coating is necessary, a light abrading or roughening of the surface is generally required. Alkyds: In moist areas, zinc will produce alkaline surface causing alkyds to saponify, causing premature peeling and flaking of the paint system, despite initial satisfactory adhesion.Due to this chemical incompatibility with zinc, alkyds are very difficult to use on galvanized surface unless the paint is specifically formulated for using over galvanized steel. Contact the paint manufacturer for more specific recommendations on using an alkyd paint system. Asphalts: Asphalts are generally petroleum-based products, which are not recommended for use on galvanized steel.Bituminous: These types of paints are thicker than conventional paint systems. As they are coal tar products, unlike asphalts, they can be used with galvanized steel. Bituminous paints are often used over galvanized steel that will be buried. Chlorinated Rubbers: Although difficult to apply, chlorinated rubbers are fast drying and provide good protection for exterior exposures and chemical resistance toacids, alkalis and moist gases. However, they chalk readily and need a high surface profile for good adherence. In addition, their high VOC content has severely limited their availability and end use. Coal Tar Epoxies: These types of epoxies are rarely used over galvanized steel. They provide outstanding resistance to acidic conditions in splash and spill areas. However, they are difficult to applyand require brush blasting or a wash primer to adhere to galvanized steel. Coal tar epoxies also are often used over galvanized steel that will be buried. Epoxies: In most cases these epoxy –esters and epoxy-amines are not generally recommended for use directly on galvanized steel as they are typically high stress materials and may react with the zinc in certain environments. However, epoxies dohave some limited success if the paint is specifically formulated for using over galvanized steel. Contact the paint manufacturer for more specific recommendations on using an epoxy paint system. Epoxy- Polyamide Cured: These epoxies generally have superior adherence to any type of galvanized surface. Because they are not resistant to sunlight, they are typically used as a primer or for...