It's common for modeling software to flip normals, causing what you describe. You said,
I only care about the externally facing faces.
But are you sure they are truly externally facing? Turn on normals in Blender and ensure that all the arrows/pointers are external.
Another problem that can generate the results you're seeing are self-intersecting surfaces, ...
I think it depends on your use case.
As Bobt already commented, Meshmixer is a common tool for editing STLs.
But also a lot of CAD-Tools should be able to import STLs.
Then there is also of course Blender.
As you didn't specify what you want to use it for, I would guess you just want to do some quick fixes to the mesh.
The tool I use is Meshmixer. It's ...
Rhino gives the following output for MeshRepair
Mesh has 3 degenerate faces. (a face that's actually a line or a point)
Mesh has 42 non manifold edges. (a line where 3 or more faces meet, possibly caused by degenerate faces)
Mesh has 6 duplicate faces. (faces that share the same coordinates)
Mesh has 148 naked edges (edges that have only one face)
All of ...
There are several problems at work here:
Internal Geometry & unjoined parts
Generally inverted surfaces: every surface is inside out!
Step 1: add missing surfaces.
Select the vertices around the hole that has no bottom surface and then pres F to create a surface. Use several steps, making roughly triangle surfaces. For ...
Okay, so like suggested I put it into 3D Builder (A free Windows 3D modeling program). I imported it as an stl, then exported it as an stl and imported that stl into Cura. And lo and behold all problems were gone! If you are having a similar issue give that a try! The 3D Builder program and similar ones are designed to export for the sole purpose of 3D ...
If you have 3D software like Blender you can import your model and use the decimate modifier to lower the number of vertices and then re-export.
Here is a link to how you can do this: Simplify Geometry with the Decimate Modifier in Blender 2.9.
This file simply is not on Thingiverse. Not all files are on Thingiverse.
A Google reverse-image-search for that picture in all size told me that the image stems from an all3dp article, and they have a reference link to Pinshape as it is presented here:
The file name of the linked picture is universal-phone-tripod-mount-3d-printing-155113.jpg
Taking that as ...
Still don't have an actual answer as to why repairing the stl file didn't seem to work, but since my stl file was a combination of different stl files I tried repairing the individual stl files before combining them, which resolved the problem.
I have been down that road with FreeCAD and STL files and never ever succeeded. I actually use Prusa Slic3r whenever I need to edit STL files such as resize, cut or split and object.
That said, FreeCAD is extremely powerful when you learn how to use it. It took me several months to get a good workflow with part design and the ability to write scripts/tools ...
The other answers on this thread seem kind of hand-wavy, so I'll give my input.
At its simplest, all we're dealing with here is two different formats of encoding the same data. The 3D file is identical, just described by the file data in different terms.
That being said, there is a multitude of different reasons that 3D prints can fail. Fusion 360 is ...