Typical surface defects are:
Poor substrate wetting
Formation of Bénard cells, floating
Poor leveling, orange peel
A key parameter for all these sorts of defects is the surface tension of the materials involved; in particular, differences in surface tension lead to the formation of surface defects. Surface tension differences can originate from the system itself (evaporation of the solvent, cross-linking reaction of the binder) or can be caused by external sources overspray or dust particles (contamination of the substrate).