Years ago, we had to replace a 42" x 108" HDU carved panel. When we got there, we couldn't tell what made it break in half. No signs of vandalism, no signs of being hit with force from a car or otherwise..... just a clean break. It was 2" think. Also, it was only a year or so old at the time and the other shop refused to fix it for free. Only thing we could tell was it was either forced in too tight and had a slight bow..... or too much bird guano buildup in the center. It had a 1/2" sintra or similar board for the backer. We matched everything completely on the face of the sign, but we used an angle aluminum 2" system and framed the back out completely and used if I'm not mistaken 'Gorilla Glue'. We routed out where the frame was to go and just glued it in and it all became one solid piece. Also, instead of using an 10' piece and then a 1' piece, we made the seem in the direct middle of the board. From the other side, you could barely tell what we used for bracing as it all became one within the panel.