Hi Michael,

Thank you for the detailled explanation.

I was now able to select the inner piece and perform the boolean difference.
Unfortunately the result of my first try was not a 'clean' Shell.

But the result of my second try (with the original Solid) was a 'clean' Shell, after:
1. Separating the Solid obtaining 10 Surfaces
2. Joining the 3 Surfaces obtaining Surface A
3. Joining the other 7 Surfaces obtaining Surface B
4. Joining the Surfaces A an B obtaing a/the Solid
5. Offsetting (as you suggested)
6. Boolean Differencing (as you suggested)

Thank you very much again
Jurgen