Create an invert for a mould

[micheus]

Humm.
If it was made with Wings3D and not imported, you need to take care with the operations you are using since it's not usual it to create so inconsistent shapes like this.

Also, I suggest you to try tessellate it on export (if possible) and keep the original Wings3D project using quads. It helps to see the mesh clearly (it has too many polys that difficult to check it).