Apparatus for treating a fibrous web that includes at least one supporting surface arranged to support a surface of the fibrous web, and at least one sealing device positionable adjacent a surface of the web opposite the surface of the web supported by the at least one supporting surface. The at least one sealing device includes a sealing element and a holder structured and arranged to hold and to enable lateral sliding movement of the at least one sealing device cross-wise to a web travel direction.