It's good practice to save & send .skp using some older version, even SU8. Online SU: Hnggggh. Cannot open.

This section is very complex. Is there an object or picture you have sampled for drawing this section?
Rather than correcting this model, it will be easier to redraw. However, it must be ensured that each piece is solid when redrawn.

1.JPG

I drew over some missing faces' edges and they healed up.
However, the one on the shallow-dome part cause a double face to form - it was only zillions of an inch apart - but stopping it being a solid.
Because the edges were supporting too many faces !
I selected one of those faces and deleted it.
Then it reported as a solid.

I suspect that there is some incredibly tiny variance in these edges which SketchUp thinks can't support a face - or can - and then adds two faces !
Doh !
These overlaid faces can some times occur - SolidInspector will show them - but can't fix them...

Excellent, glad it helped! 👍