Hello everybody,
I am looking for a method to automatically select some faces out of a mesh.
Here is the problem: I imported in blender some CAD files describing some robot parts.
I need to select only the external surface and to separate it from the rest (I am not interested in internal part details). I am doing it selecting face-by-face but it takes very long, so every advice will be appreciated 
Thank you very much!
Hmmm…
You could try to move your mouse over one of your needed faces and press L-Key.
That would select all faces that are connected to that one you aimed.
If that doesnt help that much you could try circle select BB-Key (press double) to select your faces.
When in edit mode, there is a button on the right side below your 3D view displaying a cube, the “Occlude background Geometry” button. It makes it easy to select only the faces that directed towards your view.
If you haven’t tried it yet: Blender comes with a wide range of selection options (i.e. loop select, loop→region).
Some are described here:
http://wiki.blender.org/index.php/Doc:Manual/Modelling/Meshes/Edge_and_Face_Tools
Others here:
http://wiki.blender.org/index.php/Doc:Manual/Modelling/Meshes/Selection
Good luck. If you describe your mesh in more detail we could perhaps help you some more.
Thank you, both of you
I used L to select connected faces and it worked great.
I also tried loop select but couldn’t make it work the way I wanted…but I needmore practice 
BTW, the mesh is composed by triangles, and I obtain it importing a wavefront .obj file.
The one I am experimenting comes from the forearm of a humanoid robot, and it represents the bottom half of it. Grossly speaking, it is shaped like an half pipe.
Since the object has a thickness, it is composed mainly by two layers, an external and an internal one, connected by small faces on the borders. In other words it is a closed non-convex polyhedron.
Can I add a more significant description?
Oh, triangles. In Blender you work with quads most of the time. As far as I know it’s not possible to loop-select with triangles.
You could try to convert your triangles to quads:
Feel free to describe you mesh in more detail if you need more help. Some uploaded screen shots or a .blend file would make it easier to see the problem more clearly.