A method of preparing a composite element for forming a composite assembly having a cavity is disclosed. The method includes laying a barrier layer on a surface of the composite element which defines at least part of the cavity. A support assembly including a mandrel (38) and a film bag (40) extending on the surface of the mandrel is positioned with the film bag against the barrier layer. A sealed enclosure containing the composite element is formed through sealing engagement with the film bag. The pressure within the sealed enclosure is reduced, and the mandrel is separated from the film bag while maintaining the film bag positioned against the barrier layer. In one embodiment, the element is a stringer and is positioned against a skin. An assembly for forming a composite member is also provided.