Erection or maintenance of the following signs may not be permitted in protected areas:

(a) Signs advertising activities that are illegal under State or Federal laws or regulations in effect at the location of such signs or at the location of such activities.

(b) Obsolete signs.

(c) Signs that are not clean and in good repair.

(d) Signs that are not securely affixed to a substantial structure, and

(e) Signs that are not consistent with the standards in this part.