+++If you have individual surfaces that are touching each other along common edges, you can use the Join command to glue them together. And if you have all edges in your model joined to another edge, that makes it into a solid.+++

Aah, alright, I get it. So there's no special command, but joining all the sufaces will do that.

I just had a look into my file, and it was already a solid. So this "degeneratd face" thingy probably was just something to omit or I picked some mesh settings that were too coarse.

thanks!

Sebastian