Hi Mark,

> You probably had a particular arrangement of curves for
> that, like a 4-sided boundary?

Yes, the particular extrude was four sided.

> Unfortunately that would not work in the general cases,
> it would only work if you had a 3 or 4-sided boundary.

Yes, understood.

> For now you can run Boolean Union on the result with the
> shared internal wall, it should fuse those pieces together
> and remove that part.

Jeez I'm an idiot...

I manually searched and destroyed all the internal walls I had created. If there is a hard way to do something, I'll find it!