How can I set up internal face
 

How can I set up internal faces which are not considered in calculations.
I have some annoying 'Default Faces' on the outer hull of my boundary layer (created with GMSH), where no faces are supposed to be (in between boundary layer volume and outer polyhedral volume). No clue how I can get rid of them, but defining them as 'invisible' would be a good workaround.

Regards,

Axel