Combining mesh from skin modifier to manifold object I tried to make Hair for a Character, by using the new skin modifier.



Applied to the Mesh, this produces an object with intersected Faces. I also use subdivision surface modifier with level 2.



I separated all different segments into independent objects. What I wanted to do is to apply boolean Modifiers and union all of those into a non-self-intersecting Object, which I need for 3D printing.
This works for almost all hair segments. But some boolean Unions behave like a boolean Difference. And vice versa. And in a few cases no Operation provides the needed Result. In this case, the segment just "vanishes".

 

I could really need help, to finish this project. Thanks in Advance :)
Instead of decimate, try the re-mesh modifier.  The re-mesh modifier should be able to handle all of the hair strands and their intersections.  It should act as a global "boolean union" like you want.

If this doesn't work, you could remove the boolean modifiers, export it to a STL file, and then run it through https://modelrepair.azurewebsites.net/ I've had a LOT of luck with this tool.