a method of repairing a damaged portion of a glass layer of glass - lined steel equipment by repeated steps of applying a repair agent to a damaged area of the glass layer and then heating the repair agent for solidification and adherence to the glass by the use of a sol - gel process , until a thickness of a repair glass layer becomes almost equal to the existing glass layer adjacent thereto without generating cracks or exfoliations in the glass . also , an apparatus for heating the repair area of a layer of glass - lined steel equipment includes a flexible thermal insulation sheet body having a recess on a single side thereof ; electric heater units provided in the recess for emitting far infrared rays ; magnets provided adjacent to the recess for securing the sheet body on a wall of glass - lined equipment covering the damaged portion to be repaired ; a silica glass cloth enclosing the body for maintaining configuration ; a thermal sensor placed in the recess for locating adjacent to the damaged area ; and a thermal controller coupled to the sensor and the heater units for automatically controlling the temperature of the heater units .