I downloaded a video game model. The problem is that although it appears to be a valid mesh, there are many many errors. For example, the mesh is not intact or manifold, or sometimes there's double faces, and double edges. There's many types of errors here, I think. Again it's not detectable by glancing at it. Just manually seeing what's wrong with a particular vertex, edge, or face would take forever, let alone repairing it individually. Is there a more efficient or systematic way to fix this mesh? I have attached pictures of 2 errors in the mesh. Thanks very much guys.
The operator is called
Select Non Manifold in Mesh menu. It should help you to find out weird mesh. With further configuration, you can choose what you want to inspect.
After you select those vertices, you still need to clean up them by hand or using operator again:
But don't rely on these method to fully fix your mesh since if your mesh is broken in some degree, that mesh will probably been so weird that algorithm can not figure how to fix it.
I came up with a solution. There were too many errors of different types in that mesh, so I exported the model as an STL. Then I went to Netfabb.com, which offers free STL mesh repairs, which has always worked for me. I uploaded the file and then Netfab corrected it for me as an STL, and then I went back to Blender and imported the fixed STL. And it worked. No more non-manifolds, double faces, double edges, or any other errors. Netfab has always worked for me in CAD, and now even in Blender.