This thesis analyzes the 1949 treatment of the painted plaster using polyvinyl acetate (PVAC), in the interior at the Mission Church of San José de Tumacácori. The treatment was proposed by Rutherford Gettens of the Fogg Museum and executed by Charlie Steen of the National Park Service. The PVAC was sprayed over the entire interior in order to consolidate the painted surface. The PVAC used was specifically Vinylite A, which was produced by Union Carbide. Since 2014, the Center for Architectural Conservation has been performing stabilization treatments to the interior plaster; however, the PVAC treatment has never been closely examined. The ramifications of the PVAC treatment were assessed using a combination of analytical and experimental methods. The painted decoration, which was intended to be matte in luster, now has a gloss; thus it is evident that there were ramifications of the treatment. Further analysis examines how the treatment effected the physical properties of the scratch coat, plaster, and pigment. Findings from this thesis can guide the National Park Service on future in-situ assessments of the Church plasters.