How can I prevent invisible faces when exporting to FBX file? I started recently learning how to use Blender to create 3D models, to use them later in Unity3D.

Translating the vertices of a hollow cylinder, I created the following model. Please pay special attention to the selected face.


After finishing translating all the vertices, I noticed these "faces" were accidentally deformed or even deleted. So, I deleted them and remade them selecting the corresponding vertices and hitting the Make Edge/Face button. The result is the screenshot posted here, and they look good from Blender.

However, when I export it to FBX file and insert it in Unity3D, this selected face and the one equivalent at the other side are completely invisible from any angle. I tried flipping normals but they still remain invisible.

I also tried to press CtrlShiftAltM just in case there was some undetected non-manifold geometry, as suggested in this answer from another question.

I barely can guess that this is probably because of the twisted geometry of the face itself, but I can't figure out in detail the true causes.

What kind of techniques may help to correct this issue?
This issue is due to the fact that there are N-gons in your model (an N-gon being an N sided polygon). Either FBX doesn't support these or Unity can't import them, but, either way, the solution is to get rid of them. There are many ways to do this, but the fastest is probably to select those faces and Triangulate them (Ctrl + T) and, possibly, convert those to Quads (four sided polygons) (Alt + J).

Another approach, which might get you cleaner results is to use the Bridge Edge Loops tool; this tool doesn't appear to have a shortcut, but you can invoke it either from Edges menu with Ctrl+E > Bridge Edge Loops or by pressing Space (search for operators) and typing it's name. For this tool, you should have two edge loops selected, so delete the N-gon face and select the edges that were there before (Alt + RMB (selection button)) and then execute Bridge Edge Loops.