Also, the model doesn't have to be water tight every time, it depends what you are going to do with the model, for most rendering programs a joined surface will be fine, but if want to output the model for manufacture like a into CAM package the model should preferably be a solid body or if it is just a joined surface it shouldn't have any tiny holes in the surface.

It's also quicker to model with solids in MoI, when using the boolean functions the model always stays a solid.
Working with low level modeling techniques, surface by surface, requires high accuracy and alot of practice, not only in modeling the surfaces but identifing why the model isn't a solid and knowing how to correct it.


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