The wetting process is a central feature of paint production and application. The main factor in the wetting process is the surface tension of the various components.

In order to understand the origin of the surface tension of a given material - for example a liquid - we have to examine the surface of that material on a molecular basis. In nature, the transition between two neighboring phases (in our example, liquid and air) is never a strict frontier, but a region of various molecular layers with changing densities and mobility.

Due to the extremely small size of this diffuse layer versus the bulk of the material, it is not considered further in this explanation of the surface tension.

In a simplified model the surface is reduced to a monomolecular layer. Between the single molecules several attractive forces may exist: London-Van der Waals, dipoles, hydrogen bridges and ionic forces. In the majority of a material, every molecule is equally surrounded by other molecules leading to an equilibrium of forces. At the surface however, this is not the case and therefore all forces are directed into the center of the material.

The surface tension (acting in parallel to the surface) is related to the type of attraction between the surface molecules. It is even stronger at corners and edges due to fewer surrounding layers being present. The surface tension of a liquid or a solid can be measured by determination of the contact angle. The most practical way to calculate this is using Patton's equation that takes into account the approximation of the solid's surface tension (critical surface tension).

Successful wetting can be directly related to the size of the contact angle. At a contact angle > 90° no wetting takes place and the droplet keeps its spherical shape. At a contact angle < 90° wetting improves and the contact surface (droplet-solid interface) increases. At a contact angle of 0° the desired spreading is achieved. This is only possible when the surface tension of the liquid is lower than the surface tension of the solid. For the coating industry this means that only the liquid can be influenced, as solids such as pigments and fillers as well as the substrate have a fixed surface tension.

One way of reducing the surface tension of a liquid is by adding a surfactant. Accumulation at the surface leads to compensation of tension by interaction of the polar groups. The value of the surface tension obtained is determined by the lower surface tension of the surfactant.  

This concept is the basis on which problems in the coating industry are tackled, so that pigment wetting as well as surface defects such as craters, poor flow and foam can all be addressed according to these principles.