Hey! Im having problems filling a ring. Im trying to make a difference between the ring mesh and a square mesh in order to later joining the difference or intersection between them with the ring mesh. Here some photos for better understanding 
Can you please post a file?
Yes, sure here you have the rhino file.
Yes sure here you have the rhino file
Hello - you can Mesh or ExtractRenderMesh the SubD object and use MeshBooleanDifference - in theory - but the ring mesh is messy - it has non-manifold edges. It might all work with a cleaner mesh. For now, use MeshSplit a couple of times until you get the thing split into all the necessary pieces and then Join as needed.
-Pascal
So the goal is to fill the inside of the lion ring?
Okay thank you Pascal, i will try.
Yes, how would you do it?
QuadRemesh the lion and then intersect
Yes indeed. The thing is, I also want the filling surface not to be plane. I want it to have some curve that follows the curve of the lion head inside.What would yo do? I tried mesh difference but it broked the fill i just did.
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Well it took a bit of back and forth and fixing a few mesh holes and we got this SubD …
lion.3dm (12.8 MB)
A few Rhino commands in between …
Send me a ring please 
Thank you so much for your time Martin. Really helped me your answer. The only thing i noticed is that applying QuadRemesh the model lost a lot of details, that in order to print 3D wont work that well. Anyway it really helped . Thank you!