SOMDTD,
I too am at the headliner stage. I was hesitant to use the router on the foam, but set up the router table and used a 1/2″ roundover bit with the bearing guide at the top of it. The bearing guide and the router table fence allow you to set it up so the foam can’t get sucked in too far. It worked fine although some of the small points have been a challenge. The general rounded edge is smooth. I do think it is a result of a new bit and the 25000 rpm of the router. 3000 rpm will just chop up that edge, not make it smooth-time for a new router.
I’m concerned about using that contact cement-very unforgiving! I’ll keep you posted.