This standard provides minimum recommendations for the interior lining of existing steel and fiberglass reinforced plastic underground tanks and periodic inspection of steel underground tanks used for the storage of petroleum-based motor fuels and middle distillates. These recommendations include procedures to be followed and operating conditions to be maintained by contractors, workers and engineers preparing, inspecting, testing and lining the interiors of existing underground storage tanks. Requirements for vapor-freeing tanks, removing sediment and cleaning interior surfaces of steel and fiberglass tanks as well as guidelines to identify tanks that are suitable for lining, are also included. All work associated with the interior lining and periodic inspection of underground storage tanks shall be accomplished in accordance with applicable federal, state, and local regulations, including applicable environmental and safety requirements. The methods described in this standard are applicable to steel and fiberglass-reinforced plastic tanks used for the storage of petroleum-based motor fuels and middle distillates. The procedures are applicable to tanks installed in typical retail service station outlets but may also be used for tanks installed at other types of facilities.