Hi Steve, so those lines are the surface edges - when you do a join what actually happens is the edges get glued to one another, so join will not make edges disappear the edges are still there but now MoI will have knowledge that there is a connection between the pieces on either side of the edge.

If all the edges of an object are joined between 2 surfaces that makes a fully connected skin and then forms a solid volume.

Edges are not normally things that you get rid of, they're boundaries of a surface. To get rid of an edge would mean building a bigger surface in that spot rather than having several small surface pieces.

If you want a final result that is missing edges like that, that's usually a sign that you actually want to start out with a bigger piece initially and then cut away some portions of that rather than building that particular area out of little separate surface patches.

- Michael