This of course results in about 100 naked edges occurring along the seam between the two halves. I have tried to remove the internal faces, explode and join, rebuild edges, etc, all to no avail. What am I missing here?
I think the issue was that there were duplicate surfaces along the mirror plane of the polysurface. The extra surfaces and the polysurface were also grouped together making it hard to spot. I used Ungroup and then Explode to pick and remove the surfaces that would be in the way. This left two polysrfs to Join… the outer form and the inner cylinder. Mirror and Join then worked as expected… see attached. I hope this helps. naked_edges_mirror_bjames.3dm (1.2 MB)