This specification describes a process and apparatus for closing leaks and spraying a sealant composition to interior walls of an underground gasoline tank such as is used in retail gasoline service stations. A gas tank suspected of leaks is emptied of stored gas and purged with compressor air applied thru a valve control and pipe conduit threadly inserted in the conventional fill pipe of the tank. The pipe conduit includes a means defining an aperture at the lower extremity of the conduit which extends into the tank interior. An explosion safety level meter is attached to the pipe conduit for providing a humanly detectable signal of the environment within the tank. An excavation enables a craftsperson to cut an access port through the tank wall for entering the tank interior, cleaning-up residue, sandblasting the interior walls, and spraying a sealant composition of thixotropic polyester resin and a catalyst onto the sandblasted walls.