Abstract: The process of the invention consists in smoothing the or each fluffy surface of the layer by exerting a slight friction under low pressure so that the projecting fibers are flattened on the subjacent fibrous mass without the latter being compressed, the friction being exerted at a softening temperature such that the superficial fibers are welded together to form a skin or porous glaze. The smoothing is preferably effected by means of a metal mass whose temperature is between 115.degree. and 150.degree. C. This process may be applied to the outer face of a filtering breathing mask constituted by an envelope made of this fibrous material covering a relatively rigid porous shell for protection of the skin; it may also be applied to at least one of the faces of filtering layers equipping filters of any type.