Hello out there,
now I have come across a problem in Rhino 7 Mac I’ve not had before: This polysurface is called an „open polysurface“ when I select it (pic 1), but when I run “show edges” to find the troubled spot, no naked edges are found (pic2) and „Check“ also calls it a valid object.
When I the go and try to „Boolean union“ it with another structure, this strange thing happens: in shaded mode I see only the wireframe unless I select the object … so something is obviously rotten. But what?
Would appreciate a hint!
Thank you - Jan!
P.S.: Right after posting this I had the idea to explode the thing and join it again, and it joined into two closed surfaces, one consisting only of two surfaces. The other on is declared okay, but still won’t union with a support structure.
Open??.3dm (9.9 MB)
Thanks for your swift reply, didn’t see it … right after my post I had the idea to explode and then join the thing again and found two surfaces, which then were joined into one closed surface, which I deleted. Now there are no more manifold edges, but I still can’t get the thing to union with some other polygons. I’ll keep trying 
How did you find that? Neither „show edges“ nor „select bad surfaces“ show me the trouble spot.