Abstract: A liquid sealant of the aqueous emulsion type to be delivered in foamed state into a pipeline requiring leakage repair and/or prevention. A large part of this sealant is drained from the pipeline, and the part of the sealant remaining on inner walls of the pipeline is allowed to cure. The sealant contains coagulations of weakly cohering fine particles of inorganic oxide at less than 5 percent, and the particles have diameters not exceeding 0.05.mu..

Abstract: A pipeline needing leakage repair and/or prevention is filled with a liquid sealant in foamed state of the aqueous emulsion type. Then an excess amount of the sealant in the center of the pipeline is drained from the pipeline, and the sealant remaining on the interior wall of the pipeline is allowed to cure wherein the sealant contains fine particles of inorganic oxide weakly adhering to one another into aggregations, said aggregations easily breaking into such fine particles that they can enter and plug fine cavities of the interior of said pipeline.