Is there any way of turning a joined surface into a solid? I have a shape that needs to be filled as some shapes need to go through it. I also need to join solids to it. I try to add a small solid shape to the surface but it either never joins or in boolean it just disappears for some reason.

Also, why to solids sometimes suddenly become joined surfaces during some operations so you only notice a few steps further on?

Tx Gordo