A surface coat is a clear or translucent paint coating applied as a sealer to an underlying material. In paint systems, topcoats provide a resin seal over midcoats and primers. The first top coat can actually penetrate into the middle coat, providing an impermeable top surface for the coating system. The topcoat is the first line of defense for many coatings against corrosive substances.
Topcoats are used to provide:
Corrosion-resistant seals for coating systems
initial barriers to the environment
Chemical, water and weather resistance
Toughness and Surface Abrasion Resistance
Pleasant
surface coatings are widely used in the automotive industry, where a post-coating in the painting process is required to decorate and protect the car.
Coatings in a typical coating system can be divided into primers, intermediate coatings and topcoats. Topcoats are often structured and textured and often require specialized application equipment.
Topcoats are generally denser than midcoats because topcoats are formulated with a lower pigment-to-vehicle (solvent) ratio. Common varieties of topcoats include:
Air-drying paints and oil-based varnishes, hardened by oxidation
Acrylic and other paints, drying by solvent evaporation
Polyurethane and epoxy coatings, dry by cold cure chemistry
High temperature curing or drying can also be used with certain types of epoxies to produce a harder finish, but this also makes them more difficult to remove. Whichever paint system is chosen for exterior use, it will usually be qualified by its ability to prevent filiform corrosion.
Topcoats reduce the rate at which moisture penetrates to the underlying coating. Talc is often used to improve sanding of paint prior to application of a topcoat. Topcoat systems have shorter maintenance intervals because the paint requires more frequent maintenance than either of the other options. Direct application of paint over unsealed spray-on metal coatings should be avoided.