Hi d3,

> How you turn surface to solid in MoI?
> Ok,, the model have to be watertight, but after that?

Use the Join command - once all edges of a model are joined to other edges forming a watertight closed skin, that makes the object enclose a volume and that is what makes a solid.

There is no difference between being watertight and being a solid - the way that CAD programs define a solid is by a watertight skin. This is called the Boundary Representation or B-Rep method of defining a solid, for more info see:
http://en.wikipedia.org/wiki/Boundary_representation

Often times people seem to think that there must be something else in addition to that, but that's not the case. Once something encloses a volume it can be used to define a solid where that volume is considered to be a filled region.

- Michael