The evaporation of the components, after the application of a covering enamel, changes the surface tension of the coating product since a variation of the concentration in the system is determined. At the same time the viscosity value of the applied coating also varies and ascensional currents of solvent are created from the inner coats towards the surface and this causes turbulence inside the product which influences the surface features. The most volatile and lighter particles are transported on the surface while the heavy ones, due to their different density and size, remain at the bottom. On the surface of the coating film the stains of different sizes are called Bénard cells. These cells have a high tension at the edges which tends to decrease towards the central areas.