Within the Central Business District, all building walls which are visible from a public street or right-of-way shall be finished with unpainted brick over at least 75% of the vertical wall surface, excluding areas containing glazing (doors and windows). The remaining portion of the visible building wall may be finished with stucco, split-face block, stone, finished wood, hardie board, or similar high-quality materials. In no case shall corrugated metal, sheet metal, or unfinished plywood be used as an exterior material on a wall visible from a public street or right-of-way in the Central Business District.