We investigate in this paper the physical properties and the structure of a protective barrier based on a thermal-sprayed alumina coating. This coating is known to form an electrically insulating barrier that efficiently protects stainless steel structures of melting furnaces subjected to high differences of voltage. Sealing the open and/or interconnected porosity of the alumina coating with aluminum phosphate is also known to improve the protection bymitigating leakage in the ceramic. The sealing procedure has been achieved by (i) impregnating the plasma-sprayed alumina coatingwith a mono aluminumphosphate (MAlP) solution and (ii) applying a thermal treatment to the system. A waterproof diffusion barrier presenting a good electric insulation is finally obtained and the protective performances were tested on specimens to assess their behavior under aggressive conditions. The formation procedure is characterized by parameters related to the solution and is dependent on the heating rate, the process temperature, and the isothermal duration of the thermal treatment. We show in this paper how these different parameters impact (i) the coating physical properties (thermal and electrical tests) and (ii) the coating structure that has been probedwith 31P solid state nuclear magnetic resonance (NMR).