A while ago, a friend of mine gave me an STL file from a scanner. It was a 3D bust that he wanted to print. Unfortunately, being a scan, it had some errors in it that replicator G could not deal with. However, I didn't discover the error until I told him printing out his file would be no problem. That was before I discovered the print had several errors which made a very pretty bust look like some sort of mythical spiked creature! Since the file was a scan rather than a drawing, it was impossible for me to manually repair. It is in this way that I was introduced to the concept of a "manifold" print.
One problem when making a drawing to a physical object is the fact that it's easy to draw an impossible to print object. The large errors are usually easy to find when inspecting the object in Sketchup. However, a line that is only slightly off can make an object impossible to print and such errors are sometimes hard to find. This is especially true if a rogue line or plane has formed inside an object! Ideally, all .STL files should be Manifold. Think of such an object as having no gaps, stand-alone planes or single orphan lines. Essentially, a solid shape should be water tight. Unfortunately, such objects are all too easy to draw by accident. In drawings of simple objects, these are usually possible to tease out. However, a scan may have thousands of points. So, it's practically impossible to correct such errors by eye.
Enter the NetFab program. One of it's features is the "repair" feature. Just import your .STL file. Then select the "repair" process. If there are relatively minor problems such as small gaps this program can usually find them and use an average value which is usually good enough for a print. The basic version is free to use, works well and works quick. As you may have guessed from my list of printing steps, I'll take any time savings I can get!
A while ago, a friend of mine gave me an STL file from a scanner. It was a 3D bust that he wanted to print. Unfortunately, being a scan, it had some errors in it that replicator G could not deal with. However, I didn't discover the error until I told him printing out his file would be no problem. That was before I discovered the print had several errors which made a very pretty bust look like some sort of mythical spiked creature! Since the file was a scan rather than a drawing, it was impossible for me to manually repair. It is in this way that I was introduced to the concept of a "manifold" print. 
