I used Liquid Nails to fill in holes, corners, and screw heads prior to applying the Permadri.  Do you think there's any advantage to using Bondo?  I think that VLDesign met with some problems with Bondo on both his builds.  Liquid Nails is easy to work with; just squirt it in and us a putty knife to get any excess and you're done.  Hard as rock when you're done too.