Hi Burr,

> If I shell it and then extract the back face, it then extrudes normally.

Yup, the reason why that works is that when it got turned into a solid with shell, everything got a consistent normal direction at that time since it is possible to automatically determine the "outside" of a solid, which is not really well defined for just an open surface.

When you extract faces out of a solid, the extracted faces maintain the same directions as they were in the solid.

So those steps you did basically unified the normals to a consistent direction.

- Michael