The Basilica of San Sebastian in Manila, Philippines, is one of few surviving all metal cathedrals in the world. It was built in 1891, after the previous three masonry churches on the site were destroyed in earthquakes. Originally, both the exterior and interior steel cladding panels were faux painted to create the illusion of cut stone. Trompe l’oeil figures painted by some of the country’s leading turn of the century artists decorate the interior dome and side walls. The structure is actively corroding, causing damage to the interior finishes. This study examines the original painting technique and diagnoses the specific causes of the deterioration of the interior finishes.