Abstract: A conventional undercoating method in repair painting on a damaged vehicle, which includes many working processes, possesses drawbacks of being low in working efficiency and making a finally repaired area exceedingly larger than an initial damaged area. To overcome the drawbacks, the invention provides a undercoat forming method using newly developed putty composed of specific compositions, which is capable of forming a thick coating layer. According to the invention, the processes of polishing of the coating layer by dry sanding and applying the putty respectively can be performed in one operation.