Topcoat

A preparatory coating applied to a substrate before an adhesive, sealant, coating or waterproofing membrane, improving adhesion by sealing porosity, binding dust or providing a chemically compatible surface. Primer selection depends on both the substrate and the product to be applied over it.

In multi-coat finishing systems, notably automotive painting, the pigmented coat that provides the colour and visual effects such as metallic or pearlescent flake. It is normally protected by a clear topcoat that supplies gloss and durability.

A coating defect in which ultraviolet degradation of the binder at the surface leaves a loosely bound, powdery layer of pigment that can be rubbed off. It is a normal long term weathering mode of many exterior coatings and is more pronounced in binders with poor ultraviolet resistance.

The gradual change in the appearance and properties of a material caused by outdoor exposure to ultraviolet radiation, moisture, temperature cycling and airborne pollutants. Coating and material durability is assessed by natural exposure trials and by accelerated weathering tests in the laboratory.

An additive that protects polymers, coatings and sealants against degradation by ultraviolet light, either by absorbing the radiation (ultraviolet absorbers) or by scavenging the free radicals it generates (hindered amine light stabilisers). It is used to delay chalking, colour fade and embrittlement in exterior applications.