Hi Gord, there is a script here that you can set up on a shortcut key which will select naked edges:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6051.2

That will highlight the areas where there is an edge that belongs to only one surface instead of being joined between 2 surfaces. Any naked edges will prevent the object from being a solid.

So then it's a matter of zooming into those areas and investigating why they are not joined and repairing them as needed.

- Michael