Every sign maker I know struggles with glue seams when they use multiple layers. It means instant work to get everything smooth and even. We use Coastal Enterprises PB Bond-240 - a one part bonding adhesive. It doesn't bubble up as much as Gorilla glue but there is still a glue line when we are done. I'd much rather have a little glue squish out than be short and have the joint fail later. Using epoxies will result in a very hard glue line which finishes differently than the HDU when you paint.
The obvious answer would be to route our signs from a single layer of board but that is seldom practical. It can waste a whole lot more material and it would of course be impossible to laminate in structural steel or mounting hardware into the sign.
We take a different route. First off - I really dislike sanding. While Precision Board (or other HDUs) sand up in a hurry making things perfectly smooth I believe this makes our signs look like they came out of a mold or were machine made. I LOVE my MultiCam but I don't want my signs to look like a machine made them. I want them to look hand made. We do that by purposely introducing TEXTURE. My favorite hand tool is my die grinder. It makes the hand work effortless and FAST! I keep two grinders handy with a large bit and a smaller one. Having two means I don't have to slow down much when I need to switch tools. Changing bits takes time. I also keep a new spare one tucked away in case one of these packs it in which happens occasionally. I don't oil my die grinders - which obviously shortens their life substantially. I found that the oil sprays out landing on the work... which means a paint failure down the road. The die grinders are much less expensive than my signs or the labor to produce them. So I run them dry.
When I work on the edges of my signs I first whip off the glue lines and even up the layers if things aren't quite lined up. The next step is to do any shaping that didn't happen on the router. In this case this was the rounded ends of the scroll and the fold line underneath. I could have easily programmed these shapes in EnRoute but it was simply faster and easier to do it by hand (for me). Once the glue lines were gone and the shaping was done I then quickly went over all the edges and other areas which needed texture. This included the face of the scroll. The sign instantly looked like I had spent hours and hours hand carving it. MAGIC!
Next I'll add a little MORE subtle texture in key areas like the lettering with the primer...