PURPOSE: To obtain a coating material which enables the frictional force of a joined surface to be increased when applied to high-strength bolt joints, by adjusting the particle size of zinc powder for a zinc-rich paint to a value within a specified range.

CONSTITUTION: A coating material is obtd. by adjusting the particle size of zinc powder to a value within a range of 1W143m by measuring and controlling it with scanning electron microscope, said zinc powder being used as the main ingredient for a zinc-rich paint for use in coating the joined parts of the high- strength bolt joints of steel structures. When the paint contg. zinc powder having a uniform particle size is used as a coating material and applied to the high- strength bolt joints, the adhesion of the joined surfaces of the bolt joints is improved and frictional force per unit area is increased. Thus, sliding resistance at the frictional joint surfaces of the high-strength bolts is greatly increased and a coefficient of sliding friction is improved.