I've been fighting with a three.js issue for a few 12 hour days trying to determine why some outward pointed object faces are missing. It only seems to happen if I've modified the mesh model, extruded a plane, or knife projected a hole into a mesh.
I've found a few solutions online that don't seem to be working for me: I've added in the double-sided hack for all materials and this does allow me to see into objects with holes so it is partially working. I've fiddled with different importers (JSONLoader, OBJLoader) which all seem to have the same issues as listed above, so it leads me to believe it is indeed the model itself.
The research I've seen online says that modifying a mesh can leave the normals screwed up so any faces I CAN'T see on my model viewer I just flip and do the UV Map again, but this doesn't fix it.
I'm hoping someone who knows Blender and three.js will know what the problem is. I know it's simple and I'm just missing a step because I'm new.
Here's a link to the demo site and code: http://guitar.dodgestat.com/objloader/