Hi Pilou,
3ds export is useless, as it brings in a lot of triangles.
Even I merge, the geometry is no so clean.

The best way I have found is to export as an LWO, open in modo and save as colada.
This way I have the materials, the groups....

I would prefer to skip the extra step though.
Also, when I export via the moi sketchup exporter, the surfaces are really clean, especially curved ones.
I see only the boundaries of the different surfaces as I see them in moi.