The purpose of the primer in the protective coating system
After surface cleaning, a primer is applied directly into the cleaned steel surface to wet the surface and to provide good adhesion for subsequent coatings. If primers are applied directly to steel surfaces, corrosion inhibition may be required prior to the application of the primer. There are two basic types of primers; primers that are pigmented with metallic elements anodic to steel and primers that rely of good adhesion and chemical resistance properties of the binder. Pigments with metallic elements effectively stifle steel corrosion and under-rusting of the primer until the anodic metal is exhausted. Good adhesion is achieved through surface preparation but it must be sufficient enough to prevent under-rusting in breaks in coatings that may be due to damage. Two-pack epoxy primers contain inhibitive pigments that interfere with the corrosion process.
Importance of intermediate coats in the coating system
The application of intermediate coats is to build a total film thickness of the coating system. Generally, the thicker the intermediate coat, the longer will the life since intermediate coats are designed to enhance overall protection. When highly pigmented the intermediate coating will be impermeable to water and oxygen. When laminar pigments are incorporated, intermediate coatings will reduce or delay the penetration of moisture in humid atmospheres. It will also improve tensile strength. Modern specifications for protective coating systems now require the integration of inert pigments like glass flakes to act as laminar pigments.
Finish coating process
The finish coat provides the required appearance and surface resistance of the system. The finish coat is also the first line of defense against exposure to weather and sunlight including condensation. An important factor in the protective coating systems is the compatibility of the coatings. It makes sense to obtain the coatings from one manufacturer and to follow the instructions to achieve a quality finish. Another important factor is the definition and measurement of the dry film thickness which is usually specified with minimum valued quoted. For nominal dry film thickness, individual values of less than 80% of the nominal thicknesses are unacceptable. Values from 80% to 100% are acceptable provided that the overall average is equal or greater than the nominal values.
Coating manufacturers usually provide suggestions on the right specifications for minimum dry film thicknesses to avoid excessive film thickness. If coating is over applied it can result into the formation of high stresses that result into premature failure of the system. Wet film thickness checking may also be required during application to check whether a subsequent dry film thickness will be achieved.