I'm not sure you'll want to use this on any signs actually on your property--maybe for signs you place in public areas--but I saw Liquid Ass suggested in another thread.
(Question is, how long will the stuff stay on a sign once it's applied?)
The other day I re-inforced a sign that was falling down on SE corner Gilbert / Warner (Gilbert). The next day it was gone but someone replaced it with a bigger one 2 days after that - Thanks!