The characteristics of thermosetting composite materials such as SMC are affected by the presence of reinforcements as a result of surface–matrix interactions. Surface treatments or sizings can significantly affect such interactions; hence the sizing selection may significantly affect SMC characteristics. The wetting properties of the unsaturated polyester matrix on glass fibres with different sizings have been studied by contact angle determination. In addition, the permeability of the glass fibres as well as their stiffness, the sizing solubility and swelling were also studied to understand the impregnation level of the glass fibres by the unsaturated polyester matrix before the curing reaction as a function of the sizing chemistry. The parameters that have been varied to determine the wetting and the adhesion properties provided by surface energies are the followings: (i) the sizing amount; (ii) the antistatic agent nature; (iii) the deposit method of this antistatic agent, and (iv) the film former nature. From these results in relation with the surface quality of corresponding moulded SMC panels, some of these glass fibres were selected. By using a statistical approach (principal components analysis (PCA)), it has been possible to conclude about the influence of the glass fibres characteristics on the surface quality. Correlations have also been established between the glass fibre characteristics, the material flow and the surface quality of the SMC panels.