Hi Aaron, thanks for posting the file. So your problem in this case looks like your object is not really a watertight solid - if it was then when you selected it, it would read "Solid" in the object type indicator here:



Note that yours says "Joined srf" there - that means you've got an object that is made up of multiple surfaces but it's not a closed watertight solid, there are some openings in it.

The best way to find the openings is to set up a script on a shortcut key that will display "naked edge" which are edges that belong to only one surface rather than being joined between 2 surfaces. Those are holes that need to be patched up in order for your object to become a watertight closed solid.

See here for how to set up the script:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6051.2


When you run that there will be 4 naked edges found, they are right in this area of the model:





The problem there is that you've got a sort of zero width pinched together shape in that spot, that's bad because it means the vertical edges from 2 different sides are running right over top of each other, making it a self intersecting area of the model without a clear volume being defined in that spot:



So that may need a little bit of surgery in that spot to make a cleaner shape that doesn't have a "zero width" typed pinched together zone like that. It's a little tricky here because you've got a complex model with different heights around that spot as well. But you will sort of need to decide on a different kind of shape at that point right there rather than having 2 different chunks just exactly grazing each other with edges overlapping right over top of one another.


I guess maybe the easiest approach would be to (looking from the bottom here) draw in a line like this:



Then use that to hack off some area like this:




Then possibly a couple of surfaces from the other area will need to be deleted and rebuilt, then joined in to make a fully closed solid.

- Michael