Roger  - a follow-up.. These are my notes from a successful STL repair process.. this worked for me, N.B. I use Ubuntu Linux, so I don't have access to the 3D tools on a Windows 10 machine. Stumbling around trying to fix a long broken STL model… https://formlabs.com/blog/best-stl-file-repair-software-tools/ <https://formlabs.com/blog/best-stl-file-repair-software-tools/> *I used MESHLAB 2022* Load the STL, Enable Display of Boundary Edges, Boundary Faces, No-Manif Verts, No-Manif Edges, Texture Border. Then follow the guide in the link above, but this is the report.. Apply Filter Merge Close Vertices Apply Filter Repair non Manifold Edges Apply Filter Repair non Manifold Vertices by splitting. Tried filter Simplification: Quadric Edge Collapse Decimation - failed - the input mesh isn't watertight… Apply filter Merge Close Vertices. Apply filter Close Holes.. ….. …. result!. Hope this helps.. R.