Hi Mark, there are a couple of different surfaces in that example file, are you talking about this "almost solid" piece shown selected here:




The first step for figuring out how to close something up to make a solid is to find where the openings are - there is a script that you can set up on a keyboard shortcut to show you the openings.

To do that, go to Options > Shortcut keys, and add in a new shortcut key - for the key part put in whatever you want to trigger it (I use N myself, for "show naked edges"), and for the command part paste in the script from here:
http://kyticka.webzdarma.cz/3d/moi/#SelectNaked

Then when you push N all the "naked edges" (edges that belong to only one surface instead of joined between 2 surfaces) will get selected and highlighted. So in this case it highlights these edges here:



If you zoom in to those areas you'll see that there is an opening there, also it's made up of some shorter edge pieces as well:



One way to close off something like this is to draw in some lines at the ends of those edge segments like this:



That then gives you a 4 sided boundary that you can fill in with Construct > Network - select these 3 edges and the line curve drawn from above, and then run Network on it:



Repeat that down the line and then join all those surface pieces together which should then make a closed solid.

- Michael