Hi Pilou

After planar, Moi "Types" indicates the presence of a solid.
There is a small tetrahedral solid, located by a small hole defect in the borromean-unorientable model.
I'll have to check the points involved, or fix it manually.

I do not have the skill to thicken the surfaces.
In the Costa discussion, Stefan mentions "LS3 Loop subdivision," Meshlab, "Dual remeshing," ZBrush-Z Remesher, and Fusion360 T-Splines.

- Brian