I'm glad you asked.........to cover up all of the layers, I used craft foam.......I am in the middle of painting my DMR and I used foamboard to build the body, craft foam to make the details and to cover any areas that showed the layered edges......I don't have a pic of that but I'll post one. The Foamboard is too thick and you will still see the edges of at least on slice of the FB. Use craft foam and you will hide everything. I used contact cement to attach the craft foam, hot glue will melt the Styrofoam core and ruin the piece. The same w/ the detail pieces. Contact cement.
Even added a scope and muzzle before I started painting. Be careful when building the ammo clip.....I made mine too close to size and didn't take in to account the added thickness of paint so I have to redo my ammo clip smaller in width and thickness. And yes that is the templates for a foamboard AR in the background.