Abstract: A method for applying a rust-protecting agent in straight as well as curved hollow spaces by means of spraying, either with air or airless, utilizes a flexible hose provided at its front end with a nozzle arranged to provide a radially directed circular spray field. The hose is introduced in the space to be treated and a rust protecting agent is introduced through said hose and the nozzle under such a pressure that the front end of the hose carrying the nozzle partly raises from the inner surface of the space. The hose is withdrawn through all the space.