Abstract: In a method for preparing and/or coating the surfaces of metallic hollow structural elements that have at least two connection openings between their outer and inner surfaces, first and second reaction gas mixtures (I, II) are prepared by reaction gas sources for treating the outer and inner surfaces of the hollow structural elements. The first reaction gas mixture (I) is guided over the outer surfaces and thereafter over the inner surfaces of the structural elements, and then the second reaction gas mixture (II) is guided over the inner surfaces and thereafter over the outer surfaces of the structural elements. An apparatus for carrying out the method includes a reaction vessel enclosing an outer reaction space, a central holding pipe arranged in the reaction vessel and enclosing an inner space, and hollow support arms removably attached on the holding pipe to extend radially outwardly therefrom.