Current automotive coatings systems consist of several layers, such as an adhesion-promoting layer, primer, basecoat and clearcoat. The bottom layer, i.e., the adhesion-promoting layer, plays a vital role in terms of providing better adhesion over substrates such as mild steel, aluminum and galvanized iron, and also provides a highly resistant layer to environmental conditions. In general, primers are based on different resin chemistries (alkyd, epoxy and polyurethane) and pigments (anticorrosive pigments and extenders), which depend on the application area, such as decorative or marine applications. The major ingredients in undercoats are extender/filler pigments, which vary in size and shape and are essential for undercoat properties like filling, sanding, flow and durability.
Most undercoat primer systems consist of larger-particle-size extenders like china clay, talc, chalk and barytes. These impart matting, filling and moderate resistance properties to the paint system. But these extenders also provide a less-flexible system with poor hiding properties. However, at the nanoscale level, the size and shape of nanoextenders are different and may provide better coating properties.