Hi Karl,

Watertight means the object is a completely enclosed volume (sometimes called a shell) without any holes or cracks. There cannot be any intersecting triangles, which would happen if you had intersecting volumes, however, nested volumes (one object is completely enclosed by the other) or separated volumes can be used. Intersecting triangles can also be caused by modeling operations.

There are specific tools that are used to correct objects for printing. Netfabb Basic is free and does a very good job with many common issues. However, it can't fix intersecting volumes or intersecting triangles.

If you are able to share your object, I would be happy to take a look and give you some suggestions.