3D Printing Ninja : Free Help Guide to Master 3D Printing: Non-manifolds : Manual Fixing Methods
Ways to eliminate non manifold edges? - Modeling - Blender Artists Community
How to fix non-manifold geometry issues on 3D models
Non manifold selection disappearing from menus - Usability - Developer Forum
How do I fix these manifold issues? And, should I have to "fix" the model in Simplify3d every time, or can I address that in Blender? (Blender -> Simplify3d) : r/3Dprinting
modeling - How do I solve non manifold errors with this relatively simple shape? - Blender Stack Exchange
Blender: Fixing a non-manifold mesh tutorial - Blenderheads - Mod DB
modeling - How to fix Non-Manifold elements? - Blender Stack Exchange
What is a non-manifold mesh and how to fix it - Sinestesia
What is a non-manifold geometry and how to avoid it - crwjaakko