This paper deals with the evaluation of wood − wetting process with coating materials on the basis of interactions of surface forces on phase boundary. The obtained results are compared with the actual state in the system wood − coating material. Primary ability of coating materials − spontaneous spreading over the wood surface has been proved. There is also the secondary influence of rheological properties of coating materials causing deformations of the phase boundary, non-perfect wetting of the wood surface and apparent lowering of adhesion work. The influence of rheological properties has been expressed by using the coefficient F the meaning of which follows from the comparison between results of adhesion work computed according to modified Young-Dupré equation and of adhesion work determined on the basis of the interactions of surface forces on the phase boundary between wood and liquid coating materials. A direct dependence between the values of the coefficient F and coating materials viscosity has been proved.