Hi krass, just to address your original question more directly:

> Solid and Joined Srf. - What are their essential difference?

A solid means it's a "watertight" volume, where all edges are joined between 2 surfaces, so your model has a continuous skin that divides space into separate inside and outside areas.

A "Joined srf" means that you have some naked edges in the model, those are edges that are open and only belong to one surface instead of being joined between 2 surfaces.

For example if you draw a box and delete the top face, there will be an open area on the top and it will not be a solid anymore.


> And how to convert Joined Srf. to Solid again?

You need to find where the naked edges are and construct surfaces and join them in to fill those areas in.

There is a script you can set up that can help you to locate naked edges:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3804.2

- Michael