There have been known and practically used various resin compositions comprising as the main ingredient various synthetic resins or synthetic rubbers, for example, adhesives, sealants, paints, pouring agents, and the like. These resin compositions contain usually a bonding agent in order to improve the adhesion to the substrate to be applied in addition to conventional other additives, such as fillers, plasticizers, solvents, dyes and pigments, and flow modifiers. The conventional bonding agents are phenolic resins, petroleum resins, epoxy resins, silane coupling agents, titanate coupling agents and the like. However, these conventional bonding agents are not necessarily satisfactory in the durability of adhesion. It is also proposed to apply a primer to the substrate, but this is not preferable in view of the labor saving.