I am working on a sign that is 4' x 8' x 1" HDU was going to use .5" PCV on the back to give it some strength. What should I use to bond them together? I read the post Sign info on material and "sandwiching". http://www.talkshopbot.com/forum/showthread.php?t=10919 . I was going to use Liquid Nail; but after reading this post I am not sure this is the right product. I am looking for some advice on what I should use. Thanks for any help.