The following was done to create the attached file:
- Create a cylinder.
- Convert it to a trimesh.
- Remove all faces on the side of the cylinder, only the
top and bottom caps remain (named "CylCaps"). The
normals of the top cap faces and the bottom cap faces
have opposite directions.
- Take one of the caps and turn it upside down (named
"CylCapsTurned"). The normals of all faces now have the
same direction.
- Choose edge mode for editing, use "Select Boundary",
then "Join Selected Boundaries", accept default settings
in the dialog box.
- The resulting object (named "CylCapsJoined") looks
again like a (closed) cylinder, but if you use "Select
Boundary" a second time, you will see a lot of boundaries
on the side of the cylinder which shouldn't be there. It is
therefore also impossible to assign a material to it.