Summary

When 3D modeling is focused on the render for a video or image, "if it looks good" that's all the criteria the modelers need. However, 3D printing needs to have its geometry well defined, or it can't bring something from the virtual to the real world.

Fixing bad geometry may not be the most entertaining part of 3D modeling for everyone. But being able to identify the unnecessary edges or vertices, create faces to patch holes, and rotate edges when they're in the way, and build a watertight, manifold model are valuable skills. And if you think about it like a puzzle to solve, it can be quite satisfying. Being able to take a model that wasn't made for 3D printing and fixing it, means that entire libraries of ready-made models open up online, so you don't have to make everything you want to print from scratch.

There are tools such as netfabb (http://netfabb.com) and meshlab (http://meshlab.sourceforge.net/) that can do a lot of these sorts of things semi-automatically. They're not complete...