Extract mesh "skin"

Hi !

I'm looking for a way to extract the "outer" faces of that mesh : since the mesh is closed, it is those faces which have a side facing "outside".

I tried using intersections between the mesh and little spikes normal to each face, but that seems to leave out a lot of faces that I need and pick some which are inside the mesh.(tolerance issue ?).

Does anyone have an idea on how to do this elegantly ?

Cheers,