Shear and gravitational forces applied during coating application are usually sufficient to spread coating completely over the surface to be coated. Nevertheless, it appears that under specific conditions paint can not spread out smoothly on the surface during application or immediately after application, in other words, it cannot completely wet the surface (as shown in Fig. 1).
If this phenomenon occurs in large areas or on the edge of panel or localized on part of the panel, it is called crawling. Dewetting can also results in beads of wet coating, craters, pinholes. This phenomenon depends on the surface not being well-wetted by coating.
Fig. 1: Wet coating with low tension cannot wet and ruptures on the surface
The wetting of the under layer provides necessary chemical and mechanical interaction required for adhesion of the two layers. In general, wetting of the previous paint layer requires that, overcoating layer should have lower surface tension than previous layer. Surface tension differences of a paint system formed by substrate-primer-basecoat- clearcoat layers are shown schematically in Fig. 2 .
Fig. 2: Surface tension relationship in a well-wetting paint layers.
Common causes of dewetting are ‘’dirty surface’’ due to insufficient cleaning of the surface, contamination with fingerprints or cutting oils at the paint shop. Contamination decrease surface tension of substrate to such low levels that coating with well wetting properties even cannot wet it. In order to prevent dewetting problems, it is essential that surface cleaning in paint shop should be performed.
Silicone based or polyacrylic based surface agents with low surface tension and fluorinated surface additives can be used to lover the surface tension of the paint. When selecting the appropriate surface additives, inappropriate thermal stability of the surface agent especially in oven-dry systems bears a risk of overpaintability problems (Adhesion loss on intermediate layer).