The transmissivity of metal-metal sealing joints is investigated experimentally and compared to predictions obtained by modelling. The focus is laid upon a wavy surface contacting a flat rigid part, representative of a seat-to-plug contact in an internal sealing valve encountered in nuclear power plants for instance. Experimental transmissivities are obtained from water leak-rate and pressure drop measurements carried out on a model ring-shape sample seat holding a controlled wavy defect and pressed against a rigid flat plug with a controlled normal load. The sample seat surface is manufactured by face turning a tubular part under radial stress and waviness is obtained after elastic relaxation. Modelling is performed on a 3D finite element model of the assembly, composed of the plug, the sample seat and its holder. The upper sample seat surface, which topography is recorded by confocal microscopy, is reconstructed using a modal decomposition on a basis of vibrational eigen modes. Its lower surface, in contact with the holder, is considered as perfectly flat or with its own defects. The contact aperture field between the seat and the plug is computed for a given normal load and is used to solve the incompressible Reynolds equation with a boundary element method, yielding the transmissivity. Predicted transmissivities reveal to be in good agreement with experimental data at low clamping loads and are overestimated for larger ones. Defects on the lower surface of the seat are shown to have a significant impact on the seat-to plug contact transmissivity. Nomenclature A Apparent contact area: A = π(r 2 e − r 2 i) (m 2) C contour of the contact zones F clamping load (N) h(r, θ) local aperture (m) I identity matrix K stiffness matrix (N.m −1) K measured transmissivity (m 3) K rr computed transmissivity (m 3)