Without some pic's, I'm not sure exactly what you are facing.
Depending on the material, I sometimes just scuff it up a bit ; other times I use a 'Woodpecker" to make a bunch of small holes in the foam to allow for better epoxy penetration.
Woodpecker :
http://www3.towerhobbies.com/cgi-bin...P?I=LXHZ26&P=8