In a method of treating a surface such as a paper web with a treating agent, the treating agent is foamed under pressure and delivered via a passsageway to an application zone so as to come into close proximity with the surface to be treated which may be moved continuously past the application zone. A constriction, upstream in the direction of foam flow, of the passageway causes a pressure drop in the body of the foam causing bubbles of the foam to burst adjacent the surface, being treated whereby the treating agent is applied to the surface.