It's too late for this one, but penetrol being mixed in with the paint while painting it on, will do more for your paint than any clearcoat. It will flow out, dry slower and dry harder and shinier than without it. We use it pretty much routinely.
Most paints already have hardeners and clearing type agents in them already.... if they are a good quality paint. 1-Shot LETTERING used to, but I don't have as much faith in it any more. Their clear coats were a joke and I imagine they still are.
When painting clears on top of this stuff, you're more than likely asking for trouble. If you're not matching the exact kinda paint with the clear and they have similar formulas.... one major thing will take place. They will expand and contract at different rates... especially up North here. People think because they do it in Texas or Florida that the sun is most extreme down there, so that's the ultimate test. To a degree that's correct, but the hardest thing when not matching your clearcoats to the paint being used is when it gets super hot in the summer and cold in the winter and huge temperature changes from day to night..... your paint breathes differently. When this happens, you generally get cracking. If the manufacturer has too much UV inhibitors, it will burn the paint and if not enough... it will probably flake. That's the main reason we don't mix top coats and clears.
Years ago a hand-painted sign would last anywhere from 15 to 25 years, if prepped and produced properly. Anything less was considered lesser grade signs and pretty much worthless. That's why it's so funny or odd to many of us, to wrap our heads around signs having the possibility of reaching 5 or 6 years. Again, we had lead in the background paint and the lettering paint and if you didn't thin your paint down, you did well.... and so did your customer.
Today, just about everything has been taken out of the paint and they have us using acrylic and latex paints. We'll use them, but only if stipulated by the customer. I know some people swear by them, but we still can't.
Oh yeah.... and about waxing a sign. That's an old trick to bring an old sign back to life for a few more years. Once a sign is out and starts chalking and discoloring, you simply wash it down with mild soap and water and then wax it with some decent car wax. It looks almost brand new again, but I wouldn't do it ahead of time.