To provide a coating agent for protecting a hydraulic machinery water flowing member which is improved in sand abrasion resistance and cavitation corrosion resistance, facilitates in-place workability, and allows improved maintenance; a hydraulic machinery water flowing member and a method for repairing a hydraulic machinery water flowing member.

The coating agent for protecting the hydraulic machinery water flowing member in relation to the waterwheel for hydroelectric power generation is applied to the outer surface of the hydraulic machinery water flowing member which is susceptible to corrosion and damage, and comprises a polymerization initiator which is sensitive to a light having a electromagnetic wavelength of 180-500 nm and a radiation-curable compound having an ethylenic double bond. The method for protecting the hydraulic machinery water flowing member comprises coating the coating agent for protecting the hydraulic machinery water flowing member to the outer surface of the hydraulic machinery water flowing member and photopolymerizing to form a corrosion resistant layer.