Alkyd resins, based on unsaturated fatty acids, modified oils or epoxy ester resins, while in contact with air reactively crosslink to form films resistant to attack by water and other air pollutants. The slow auto-oxidation reaction can be accelerated by using driers based on salts of certain metal ions.
The formation of a partially cured surface film can unfortunately start prematurely during production of paints and during bulk coating storage due to the film's reaction with oxygen in the air. Although this skinning process inhibits air from penetrating deeper inside the paint, it also creates problems during application and promotes defects in the film's final appearance.