What Does Top Coating Mean?
Top coating is a transparent or translucent coat of paint applied over the underlying material as a sealer. In a paint system, the topcoat provides a resinous seal over the intermediate coats and the primer. The first topcoat may in fact penetrate into the intermediate coat, thus providing the coating system with an impervious top surface. The topcoat is the first line of defense of many coatings against aggressive agents.
Top coating is used to provide:
- Resistant seals for coating systems
- Initial barriers to the environment
- Resistance to chemicals, water and weather
- Toughness and wear resistance to the surface
- Pleasant appearance
Top coatings are extensively used in the automotive industry, where the final coat in the painting process is needed to both decorate and protect the automobiles.
Corrosionpedia Explains Top Coating
The coats in typical paint systems may be divided into primer, intermediate coats and topcoat. Topcoats are generally structured and textured, often requiring specialized application equipment.
Topcoats are generally denser than intermediate coats because topcoats are formulated with a lower pigment-to-vehicle (solvent) ratio. Common varieties of topcoats include:
- Air-drying paints and oil-based varnishes, which harden by oxidation
- Acrylics and other lacquers, which dry by solvent evaporation
- Polyurethane and epoxy paints, which dry by cold-curing chemical reactions
High-temperature curing or stoving can also be used with certain types of epoxy to produce a harder finish, but this also makes them more difficult to remove. Whichever paint system is selected for exterior use, it is usual to qualify the system on the basis of its ability to prevent filiform corrosion.
Topcoat reduces the rate of moisture permeation to the underlying coats. Talcs are often used to improve the sanding of the paint prior to the application of a topcoat.
The topcoated system has the shortest maintenance interval, since the paint requires more frequent maintenance than either of the other options. The use of paint directly over an unsealed sprayed-metal coating should be avoided.