PURPOSE: To hermetically seal a C body of a single material with a coating material having a coefficient of thermal expansion close to that of the C body, by applying a vacuum porous C body with a coating compsn. comprising a soln. of a specified polyamic acid filled with C and heat-curing the coating to conduct imidation to enable the C body in the coated area to be held in vacuo.

CONSTITUTION: A coating compsn. comprising a soln. of a polyamic acid, pred. by reacting pyromellitic acid dianhydride with 2,2-bis[4-(aminophenoxy) phenyl]hexafluoropropane, filled with C (e.g. graphite powder) is applied to a porous C object in vacuo, and the coating is then heat-cured to imidize the polyamic acid to a heat-cured polyimide, thereby enabling the C object in its coating area to be held under vacuum.