SignFoam bonding 2 pieces together

use glue made for it. Coastal Enterprises PB bond

 

Gorilla Glue will do a fantastic job too. Just make sure you clamp it well, since that stuff expands like crazy, and wipe off any excess before it dries or you'll be sorry.

 

really depends on what you intend to do with the seam afterwards. Epoxy, PB bond and gorilla glue are harder than the HDU itself so sanding across the seam leaves a bump but work well if your leaving it alone. If your carving or machining across the seam, expanding foam spray works well - needs lots of clamps or weight but its easy to machine

 

thanks for the advice.
the crack is not seen when the sign is concaved. when its convexed, there it is! i think to min. my "glue expansion issue" is to drill small pilot holes in the back of the sign, near the crack and apply my choice of adhesive with a srynge. since you will not see that part (back), and use west marine 404 epoxy. i rather not deal with the expansion issues with gorilla. i am adding a eight foot aluminim brace along the back. this should add strength. and help set the sign. the L bracket is a 2"x2" quarter thick aluminum bracket.
we shall see.....

 

Some will recommend any old thing.

And some will go ahead and use some concoction not made for it gluing HDU. Then when you cut into a lower layer through the glue line ,,, the glue will show a section line. Almost impossible to hide when trying to make a perfect smooth finish.

Use the right stuff and you will have the right stuff.