A method for preparing a composite filter medium (1), which involves a step of forming a first filter medium (8) by depositing nanofibers (4) on a base fabric (2) by means of an electrospinning process, a subsequent step of forming said filter medium (1) through plasma deposition of a coating (7) on said first filter medium (8) and a step of forming irregularities on the surface of said coating (7), through plasma treatment of the filter medium (1) obtained in the previous plasma deposition step. With respect to the known filter media, that of the invention offers the advantage of maintaining the desired level of water and oil repellency, without interfering with the secure adhesion of the filter medium to the body on which is located the opening to be protected against liquid infiltration.