Pressure marking (or pressure mottling) is an uneven gloss pattern on the surface of a prepainted coil. It is generally caused by dissimilarity in the gloss and in the roughness between the top and bottom surfaces of a coil. These gloss differences are transferred with time and pressure to either surface. Pressure marking is not a degradation of the paint surface except in extreme conditions. The irregular pattern is typically temporary and will dissipate with time and exposure to heat or ambient temperature.
In the mechanism by which pressure marking occurs, the gloss components, which are under normal suspension within the topcoat and are random in directionality, are suppressed or flattened within the topcoat under the pressure of coiling.
Coil coaters use various best practices related to the backer specification, rewind tension and coiling temperature in order to minimize pressure marking.