I’ve noticed that I Rhino cannot properly section this. It seems valid; no gaping holes leading to another dimension.
I can upload the file to McNeel, but not public, this time.
Moved to Rhino for Windows,
Edit: ‘Housekeeping’; just to get your post indexed and answered 
Hello - what does Check say about this thing?
Can you post or send us the file at tech@mcneel.com? Please include a link back to this conversation in your comments.
-Pascal
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Sorry Fred.
Hi Pascal. The thing checks out. It renders. I can even get a volume calculation on it.
I sent it.
I am working on an finned cylinder-maker in Grasshopper.
Thanks,
BrendaEM
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I think you can try with DivideAlongCreases with SplitAtTangents=yes to split the surfaces into simpler ones. Then try with section
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Thank you Diego.
did it works?
Yes is does.
I’ve…not encountered that command, until now.
Sometimes, like when filleting, it’s the extra splits that run tangentially that cause issues, so I am often more worried about introducing splits than adding them.
Rhino is actually a dauntingly large program. When I first started with it over 10 years ago, I ran through every tutorial they had. All of Rhino’s some 750 pages of stuff, and all of the late Broadway Bob’s Rhino 3D tutorials, I think his name was. To get caught up on Grasshopper, I did 120 hours in a few sets of tutorials, so I am still a newbe. Still, in vanilla Rhino, there are commands luring in dark corners I am sure I haven’t seen.
Thank you Diego.
And thank you Pascal.
in this case, your input curves already have the fillet, that’s why the surface doesn’t have the theorical edges on the creases. and that leads to problems like that.
the most common case is using Revolve with curves already filleted. like tire modeling for example. this also leads to meshing issues that are visible in renders.
In your case I think you used RailRevolve for the ribs. is mainly the same problem.
the next time you use revolve or rail revolve, leave the fillet to the outcome surface, not to the input curves.
Basically, I am writing a Grasshopper…um–sketch to create the cylinder synthetically from a point. For this purposes it’s a great advantage to have as much done upstream as possible, without having to select multiple surfaces for filleting–on multiple versions.
I did run across this case where Section failed.
The new Grasshopper fin maker will use Sweep1, and would likely create the same Section issue.