Any sign existing at the time of the passage of this chapter and which violates any provision thereof shall be deemed a nonconforming use and may be continued, maintained and repaired upon the present premises or location; provided, however, such sign was lawful under any prior ordinance. Any sign unlawful under any prior ordinance shall remain unlawful unless it complies with the provisions of this section and there is issued by the Construction Official a sign erection permit therefor.