Large (3 x 3 x 0.05 m3) refractory pieces (as the ones used for examples in smelters or incinerators) do not sustain regular glazing in a kiln, mostly due to high associated costs. Still, glass coatings could find use on such pieces due to their physical properties (durability, chemical inertia, tightness, etc.). Thermal spraying, using oxyacetylenic flame in particular, appears as a cost-effective solution permitting to circumvent the aforementioned disadvantages. This study aims at evaluating the quality of two types of coatings in terms of permeability. The first type considered coatings (resulting from a previous optimization of the spray operating parameters) sprayed directly on the substrates whereas the second one considered an additional brass underlayer manufactured by twin-wire electric arc spraying. The wettability of the glaze on the refractory substrate and on the brass underlayer was studied to comprehend the coating structural attributes (thickness, porosity, crazing, etc.) as well as their effects on the permeability. A specific measuring device was developed to assess permeability.