for 0.21.2
- import the STL file
- switch to Part part workbench
- select the imported part, part menu > create shape from mesh, in there: [x] Sew shape, set to 0.01mm
- delete the original imported part
- select the body created in the previous step
- part menu > convert to solid
- delete the earlier part
- select the solid part, part menu > create a copy > refine shape
- delete the original part
based on this yt video: https://www.youtube.com/watch?v=dr1qtaURrvI