To provide a primer coating film of an unsaturated polyester resin which reinforces a woody base material and shows outstanding conduit sealing properties and further is smooth without gloss by applying the primer coating of an unsaturated polyester resin to a base material and irradiating the surface of the coating film with active energy radiation after the coating film is internally cured.

A primer coating of an unsaturated polyester resin containing an organic peroxide, a metallic salt of carboxylic acid and a photopolymerization initiator is applied to a woody base material, and the coating film is internally cured at normal temperatures or under heat, and further the surface of the coating film is cured by irradiating it with active energy radiation. In this case, the metallic salt of carboxylic acid is preferably a cobalt salt of higher fatty acid, especially favorably a cobalt octeate. In addition, the equilibrium moisture content of the woody base material is preferably 3-20 wt.% and the air-dried density of the woody base material is preferably 0.30-1.90 g/cm3. The active energy radiation is preferably ultraviolet rays. Thus, the upgrading of the surface hardness and the abrasion resistance of the coating film of the unsaturated polyester resin is achieved.