Materials shall whenever possible be in keeping with the brick or stucco finishes of the original exteriors. Materials other than these original materials such as aluminum, steel, or vinyl siding, asphalt or asbestos siding, finished glass, porcelain enamel, terra cotta, imitation brick or stone, corrugated metal, plastic, glazed or bright metal, among others, are not recommended except in those cases where particular finishes have acquired significance in their own right.