Structural sandwich?

[kurt_rose]

Hi guys. Just finished the first side of a 2 sided sign I'm doing out of Duna Corafoam. The stuff is great to work with. This is my first real full sized commercial sign and my plan is to put a piece of 3/4" plywood in between and gluing it together with Gorilla glue. Do you think pressure treated plywood as the structural element would be OK or should I go with MDO. I plan to cap the top and bottom with 3/4 PVC and running the plywood 1/1/2" long on either side so I can set it in a grove plowed onto the 6x6 posts and then caulking it in with Lexel. The sign is 90" long. I'm really looking forward to your guys input. Thanks in advance. PS> I'm waiting on some Modern Masters iridescent gold paint for the letters. Feel free to critic.

[blackhawk]

Kurt,

I have never tried gluing HDU to MDO, but anytime you glue two different materials together you can have problems. My idea would be to use some flat metal bars in the middle of the two halves (horizontally) to support the sign. You could route a pocket in the back of each sign half to accept the flat bar. Then glue the two halves of HDU together around the flat bar. Of course, you want to make the flat bar longer than the sign width so that you have something to mount to your posts. This way you have HDU glued to HDU so it will move the same and hopefully not break the glue joint. The flat bar is doing all the work and the HDU is just sitting on there looking pretty. If the sign is large and heavy you could also step up to square or rectangular tubing instead of the flat bar.

[letterman7]

You'll be fine. I've done the same thing for years with no issues. I tend to overkill my builds, so I will add a handful of stainless short deck screws through the HDU into the ply or MDO - I prefer the MDO as it's a little more stable than the pressure treated ply. I've not capped with pvc before - that's a good idea. I usually finish the edges with either a paintable caulk or generic bondo for a smoother edge look if it'll be seen up close all the time.