I found out what the problem was. I found the answer in the Border Edges thread where jsprogg pointed out "You will not be able to merge two meshes together unless you have a vertex placed at the...

Backface culling is off and the normals are pointing out. If it was a backface culling issue wouldn't the faces still be visible from one side?

I just tried it but it didn't help. The faces are transparent from both sides, whereas all my other faces are currently visible from both sides.

Hi. I've combined two meshes into one using Mesh, Combine and then joined them using the Append to Polygon Tool and merging some vertices. Then I used the Cut tool on another part of the mesh. ...