Zinc-rich primer is a liquid coating that features microscopic solid zinc metal spheres held together by resin. There are various options for the type of resin that can be used. Most organozinc-rich primers contain epoxy, although polyurethane and vinyl are available. A variety of silicate-based inorganic zinc-rich primer resins are also available. This article focuses on solvent-based ethyl silicate zinc-rich primers, which are commonly used inorganic zinc-rich primers. SSPC Coatings Specification No. 20, Zinc Rich Coatings Type I - Inorganic Coatings and Type II - Organic Coatings, is a test procedure for evaluating the protective properties of these coatings.
Surface preparation and application of inorganic zinc-rich primers is similar to other protective coatings. The minimum surface preparation for application of an inorganic zinc-rich primer is SSPC-SP 6 / NACE No. 3, Commercial Blast Cleaning; although SSPC-SP 10 / NACE No. 2, Near White Metal Blast Cleaning is recommended for harsher environments. In general, angle abrasives are recommended to produce greater peak density. The application of inorganic zinc-rich primers is more challenging than typical protective coatings. Liquid inorganic zinc-rich primer is impregnated with dense metallic zinc pigments (up to 90% zinc in dry film). Pigments are usually supplied as dry powders and sieved into the liquid components while stirring with mechanical mixing blades. However, once the zinc pigment is mixed into the liquid component, it will settle out once mixing stops; therefore, continuous agitation of the coating is recommended during application.
Zinc-rich primers have a relatively low resin-to-pigment ratio; therefore, they tend to spray dry if the application technique is poor, or if there is air movement (wind). Additionally, inorganic zinc-rich primers have a tendency to mud crack if applied at too high a thickness. Typically, inorganic zinc primers are applied in the range of 2-4 mils (50-100 μm) and often produce mud cracks at thicknesses as low as 6 mils. Because of these and other challenges, application of inorganic zinc-rich primers is typically done in plants where air movement can be controlled, and the surface to be coated more easily controls the thickness applied. They are rarely applied in the field.
