Disclosed is apparatus for simultaneously subjecting the inside and outside surfaces of a procession of containers to a gaseous treatment, such as the hardening by heating of a previously applied liquid coating. The containers each provide a closed end and another open end by which it rests on a conveyor which carries a procession of the cans through a treatment region. Structure is provided for the creation of a difference in air pressure as applied to different portions of the open ends of the containers to induce air currents interiorly of thereof.