Lot of good suggestions here. I already knew the "right" answer, obviously the best thing to do is to cut out the bad metal and weld in good metal.

But I don't have a welding rig, and I don't see me dropping the $$$ to pick one up anytime soon.

The second best answer was to cut a piece of metal, and solder it in place. My soldering iron is only about 35W, so I don't think I'll be able to do that, either.

This leaves me with the "Liquid Nails" option, which I think is probably how I will proceed. Does it have to be a street sign? Because I think that I can pick up a couple of sheets of heater duct metal and it'd work just as well.

Honorable (dishonorable?) mention goes to the guy who suggested squirting in the expanding insulation...I got a laugh out of it, but I can't see doing that. (Although, to a few elitists I'm sure that my bondo idea is the same thing. Take note: not all of us are professional bodymen, and for some of us, this is the first time we've tried this sort of project.)