The low molecular weight (primary) amines from a curing agent used in high-solids epoxy coatings are typically hygroscopic. They tend to easily migrate from the body of the coating to the coating surface, where they react with the carbon dioxide and moisture from the air to form ammonium salts.
This phenomenon is known as carbamation, blushing, or sometime as amine exudation. This is a process that “competes” with the amine-epoxy reaction, as instead of the normal chemical reaction between the amine with epoxy, the amine reacts with carbon dioxide and water. It is favored at lower temperature and high humidity and manifests itself as haziness, reduced gloss, or white crystals at the surface.
Carbamation is highly undesirable because it not only diminishes the visual aspect of the coating, but may also lead to poor intercoat adhesion, as it may obstruct adhesion of the paint applied over paint affected by amine blushing. That is why when a curing agent is designed, special attention is paid to this aspect to insure its tendency to blush is minimized.