Waterborne coatings form films through a multi-stage process.  After application by brush, roller, or spray, well-dispersed polymer latex particles are randomly distributed across the substrate.

As drying begins, most of the water evaporates, leaving a closely packed array of particles with the remaining water trapped in the interstitial spaces between them.  Continued evaporation generates capillary forces and osmotic pressure that pull the particles together, deforming them into a tightly packed hexagonal honeycomb structure.

When the temperature exceeds the polymer’s glass transition temperature (Tg), the particles coalesce and the polymer chains interdiffuse and entangle forming a continuous, clear, cohesive film.

This mechanism is why latex emulsions are so effective; they deliver extremely small polymer particles (0.1–0.5 microns) dispersed in water at relatively low viscosity.  Each particle contains high molecular weight polymer chains stabilized by surfactants.  The dispersion appears white because the particles scatter light.