Hi SurlyBird, also one thing that helps with working with solids is that when you know your object is a solid you know all edges are joined.

One kind of error that can cause problems is if you have surfaces that don't properly touch each other in all areas, making some gaps and unjoined edges maybe where you don't expect.

It is possible in many situations for pieces to touch each other fully in other spaces so that an object will select and be connected together but not in all areas.

Here's an example:




So notice there that those pieces touch each other in other spots, so that they would select all together. But there is actually a gap there indicated in red - now imagine that gap is smaller in size than that so it is not quite as easy to see immediately when zoomed out.

That kind of non-joined edge can cause other problems, like you can't properly fillet edges that are not joined to one another, and a mesh generated there won't necessarily have unified vertices there, etc...

Working with a solid can just be a kind of way to keep clean model hygiene, and let you know that you don't have any unjoined edges like this.

- Michael