Hi Lars, I just sent you a PM as well - I think the "secret step" in this case will be to not use Shell to create your final surface.

Instead use Offset, but don't attempt to offset the surface - it will not create the kind of edges you want since it will follow the surface normal as I wrote previously.

Instead offset each of the curves that you used to Loft your surface in the first place. Then loft a new one through all of those new offset curves.

This does not create an offset surface - and actually the outside and inside surface will touch each other along the vertical axis line.

But it seems to be pretty close to what you are after:



This seems like a better strategy to pursue.

- Michael