The subject invention relates to a method of applying the hydrocarbon barrier inside plastic fuel tanks. The plastic fuel tanks are supported on a carrier in an upright position for movement in a loop through inverted positions. While upright, hot and dry air treats the inside of the plastic fuel tanks followed by injecting SO3 inside the plastic fuel tanks through the nozzles. The SO3 is extracted from the plastic fuel tanks followed by injecting ammonia into the tanks through a second nozzle. After extracting ammonia from the plastic fuel tanks, the tanks are inverted and drained followed by a rinsing with a high pressured water and drying with hot air.