Hi Dirk - are your surfaces joined together so they share a common edge?

For the resulting meshes to be welded, surfaces need to share a joined edge, so that may be the problem. If the surfaces are totally separate objects that are just next to each other, then that can lead to a lack of vertex alignment between them when they are meshed.

If that's not the problem, could you please post a .3dm file (or e-mail to me at moi@moi3d.com) so I can take a closer look at the actual model data?

- Michael