Recently I sent an IGS file out to get a manufacturing quote on a conceptual design. The vendor balked because it was not a solid model but surfaces. High on my wish list is a command in Rhino that would turn the model into a true solid.
The IGES format does not support solids - at least the last time I was using it extensively. If you want to retain the “solid” information (surface joining) export as STEP instead.
–Mitch