Surface-treating method of screw parts

A surface-treating method of screw parts having threads, which method comprises:

• coating a one pack type, heat curable liquid organic resin based coating composition having a solid content of 3 to 20% by weight, a viscosity of 10 to 500 centipoise and a thixotropic index of 2 to 7, and capable of forming a film having a volume resistivity of 1014 Ω·cm or more at normal temperature, completely covering thread surfaces only of screw parts, said organic resin based coating composition comprising (a) an organic resin based composition containing a hydroxy group-containing fluoroacrylic resin prepared by copolymerizing comonomers of perfluoroalkyl (meth)acrylate and hydroxy group-containing vinyl monomer and having a fluorine atom content of 10 to 40% by weight, a crosslinking agent, and a thickening agent, or (b) an organic resin based composition containing a modified epoxy resin prepared by substituting a primary hydroxy group for epoxy groups at both molecular chain terminals of bisphenol type epoxy resin and having a number average molecular weight of 3000 to 10000, and a thickening agent;
• heating and drying said coated screw parts to form the film having a uniform film thickness all over the thread surfaces only;
• mounting said heated and dried coated screw parts by welding onto automobile parts selected from the group consisting of body, chassis and suspension;
• and
• applying a second coating composition consisting of an electrodeposition coating composition to the automobile parts, said liquid organic resin based coating composition being coated onto the thread surfaces only of screw parts by a method which comprises dip coating the thread surfaces only of screw parts in a vertical, longitudinal position;
• and said heated and dried coated screw parts showing improved properties in fastening properties, spatter adhesion-preventing properties, and thread crest coating properties.