Step 4: Repair Your Model with NetFabb
I should preface this step by saying that I know very little about NetFabb. My problem was that I needed a way to repair a mesh to make it watertight for 3D printing, and NetFabb was the solution. There are several programs out there that can accomplish the same thing, but when I googled "repair mesh watertight" this is the one on which I settled. That being said, I've found NetFabb to be simple, intuitive, and powerful, a homerun when you're looking for free software.
The goal of this step is to fill in any holes in your OBJ file that make it not watertight. Just in case anyone is wondering what "watertight" means, it's exactly what it sounds like; if you were to fill your OBJ file with water, would it leak out? The most likely places you'll get holes are at the top and back of the head, where the complex texture of the hair causes problems with 123D Catch, and at the bottom of the neck where the photos cut off.
Open up NetFabb and go to Project > Open. Navigate to and select your OBJ file. You should now see your model in the NetFabb window. Use your right mouse button to rotate the model around and check it out. You may notice some holes at this point, almost certainly at the bottom of the model, but we're going to repair both the holes you see and those you don't.
Go to Extras > Repair Part. You should see your model turn blue and wireframe, with invalid surfaces colored red and unclosed edges turned yellow. At the bottom right of the window click the button labeled "Automatic Repair". A window will pop up asking if you to select either default or simple repair; either one will do fine most of the time, but if one of these options gives you a problem the other might help. After choosing your repair option you'll notice that all holes, even very large holes, have been closed. Make sure to click "Apply Repair" after selecting your repair options. Go ahead and choose yes when asked to remove old part.
Export the part by going to Part > Export Part > As Wavefront OBJ. This will save our model as an OBJ, the same format with which we started and the easiest format to open in MeshMixer. This is the same process we will use to eventually convert our model into an STL fileformat, the standard for 3D printing. If you'd like you can go ahead and export your head as an STL now.
You now have a watertight model, suitable for printing! At this point we could skip to Step 7 and print our head as is, or continue down the rabbit hole and see where the wonderful (and sometimes infuriating) world of model editing takes us.