a mixture of steam and air alternatively the sealant is introduced into the leaky conduit by means of water the sealant being a finely divided inert material the conduit is emptied and compressed air charged with particles of an abrasive substance is blown through it the conduit is then internally coated by blowing into it a solvent free epoxy resin containing a hardener and fibers application_number US 18644094 A description this application advantageously occurs with those system portions which either comprise a main line 1 02 from inlet 1 12 to outlet 1 14 and supply conduits 1 05 to 1 08 with the corresponding openings 1 15 to 1 18 or a main line 2 02 from inlet 2 12 to outlet 2 14 and supply conduits 2 05 to 2 08 with the corresponding openings 2 15 to 2 18 when blowing out such system portions the compressed air connections are arranged at 1 14 to 1 18 such that blowing is carried out in a co current or counter current these connections being actuated by means of change over valves during the introduction of resin the resin composition is advantageously introduced into the blown out conduit in lump or atomised form in predetermined quantities via the change over valves and is blown through by compressed air the apparatus for carrying out the above described sealing and internal repair process by means of air comprises in addition to a mobile air compressor installation an apparatus for charging the compressed air with abrasive substances an installation.