For a while now, I've been using the gray Gold's Gym puzzle mat (4# EVA) from Wal-Mart for the box on just about all of my weapons. Sometimes I route it, sometimes I do a regular box build, depending on what I'm building. I've never sanded down the ridges on it.
The problem I'm having is getting good, lasting adherence when DAPing it. It seems like I'm using a lot of DAP, but the last few weapons I've built have had core separation within a month or so of use.
Are the ridges on the foam causing this problem, or is there some trick to dealing with 4# foam?