AbstractThere is some controversy about the significance of the wettability of the unfilled resin bonding agent that is generally recommended for prior application when placing a composite resin restoration. It has been suggested that the degree of viscosity does not matter. However, when investigating the union which is developed between composite resin and glass ionomer cement in the ‘sandwich technique’, it was noted that when using certain composite resins and their prescribed bonding resins the failure under tensile stress was adhesive at the union rather than cohesive in the cement. Further investigation showed that the bonding resins involved in these failures were more viscous than those bonding resins used in the more successful unions.In this investigation the contact angle of the bonding resin on the surface of glass ionomer cement was used as the measure of viscosity. It is suggested that the degree of viscosity of the bonding resin is significant in the success or otherwise of the ‘sandwich techn