The physical drying, coalescence
This applies to water-based paints in which the binder is a polymer dispersion in water. When the water evaporates, the polymer particles are forced to close. When all the water evaporates, the polymer particles interpenetrate to form a uniform film. In the case of Pébéo acrylic paints for decoration, and acrylic paints for artists, or Acrylcolor for Artistic Awakening - once the film is properly formed and coalesced, it becomes insensitive to water or waterproof.