Hey!
I made this surface with sweep1 but for some reason when I zoom in on it then it has an extra “invisible” layer on top of it. But when I select the surface then it shows that the surface is good. And when I put it to revit then it still uses this “invisible” layer. How to get rid of it?
Most likely this is just a low polygonal rendering mesh. You can assign a custom mesh from the object properties panel, or set global custom mesh settings from the Rhino options.
Thank You very much for the fast reply! Basically it fixed the issue in rhino when I changed the max aspect ratio from 0 to 1.
BUT
The problem is still there when I export it from rhino to revit.
Can You please tell me how could I import it to revit correctly as well?
The custom mesh settings are used in Rhino only. Once you export the NURBS geometry to other file format, the mesh settings no longer affect the rendering mesh of the 3d models inside.
However, you can export the render mesh, in case that you don’t need NURBS geometry in Revit. You can do that by selecting the NURBS models and run the ! _ExtractRenderMesh. Then you can select all mesh models either via the '_SelMesh command or the '_SelLast command.
As for the global render mesh setting where you changed the aspect ratio from 0 to 1, keep in mind that this particular number tries to keep all render polygons square, so the mesh is very dense and may become too heavy for your video card. Depends on the complexity of the whole scene. If you have such issues with too heavy meshed, I recommend you to use a value of 3 or 6 instead of 1 to achieve a faster framerate.
Thank You very much for Your time and effort to help me!
Yep, this way it creates it and it is good in revit. But I forgot to mention that as a mesh it is useless, because it needs to stay as a polysurface (solid) because it needs to be able to be exploded later in AutoCAD for many other drawing purposes. Do You know any other way to make it work than without making it a mesh?
You can split the problematic surface into several surfaces by its isocurve (make sure that it’s not joined to the rest surfaces while doing that).
This will force both, Rhino and Revit, to generate a more dense mesh. 
But the downside of this approach is that this surface will be no longer a single piece. If you split the surface without shrinking it, it will preserve its original structure and could be untrimmed again in the future.
These issues with rendering mesh occur when some surface is too long but thin. Dividing the long surface into several shorter ones eliminates the problem.
An alternative is to use the ! _RebuildUV command and set “Point count=4” along the U direction (vertical direction in your case). That will add some more control points vertically and usually helps with similar mesh issues, but not always. the benefit is that it preserves the control point structure along the V direction and, if needed, you can always remove the extra added control points.
WOAH! You are an absolute angel for putting so much time and effort to help me with this issue <3
You even made a YT tutorial, that’s crazy! 
I am very very grateful! Thank You so much!!!
I’m glad to hear that this solved the issue with rough render mesh.
