Help with fixing/reversing/deleting normals on the inside of 3D models

So I’m new to SketchUp and I’m also learning other CAD programs like Blender and Mesh Mixer.

When I import an .SKP models from 3DWarehouse and I try to fix the reversed normals on the inside and nothing seams to work.

To date I’ve tried using addons like Fedo Tools, Fix-It 101, Ithil_facefinde_reverseface_v1.0 and thus far nothing seams to work even though the addons say that they fixed hundreds of normals nothing actually changes.

Is there a native method in SketchUp to switch ALL the reversed normals to outside facing? Could it be something simple that I’m missing like exporting vs saving the file?

I attached an .SKP file for you to look at.
Thanks in advance for any help

mercedes+300+slr+ulenhaut+coup_Open.skp (2.1 MB)

The model you attached appears to have all the faces correctly orientated.

Thanks for the reply. I realize this is a SketchUp forum, but after exporting the repaired file as an .STL then loading it in Blender the face orientation setting shows that the faces on the interior of the model are shaded in RED which means they’re inside facing?

There are still a few holes in your blue surface[s] - these need healing…
But the main issue is that your object consists of one ‘surface’, so it can’t ever be a solid, because the faces forming that surface will always have a visible back [red] side: and of course it has ‘edges’ with only one face associated with them, so it’s never going to be manifold without work…
If your aim is to make a 3d-print of it you need the ‘extrude’ the surface[s] into a 3d-solid.
There are extensions to help in this - like JointPushPull by Fredo, but the effort required to fix this completely will be considerable !

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So is it safe to say that all or most .SKP files on 3D Warehouse are created with correct facing normals only on the outside and they can’t be used to print 3D objects from? Seems like I’m missing something obvious that would fix them all so they would be manifold (waterproof) inside and out.

I think you are missing the point that models of car bodies are typically modeled as a single skin. There’s no actual thickness. In order for the thing to be 3D printable, it has to have thickness. In order for an object to be consider solid or manifold, every edge must be shared by exactly two faces. So no stray edges, no holes in the surfaces, no internal faces.

Here’s a section cut through a randomly selected car body. I’ve hidden stuff like the interior.

The shell has no thickness to it and it’s not a closed shape.

Thanks Dave for making this more understandable. So are all these models that are lacking thickness and that are single sided able to be made printable if I had a way to add thickness and create two faces ? I’ve tried lots of addons but maybe I need a new approach.

They could be made printable but at least with the example I showed, it would take a fair amount of effort. Whether that is a worthwhile investment of your time is another matter.