VrayMTL: Doube-Sided does not work on specific object

[kosso_olli]

Hey guys,
I ran into some trouble with CAD data we got delivered. This is not uncommon, so I have seen a lot of problems over the years. So far there always was a solution, but now it's different: I have an object where some parts of the geometry have flipped normals. Because this happens on all the parts in the scene, flipping by hand is no option. So I thought I could use the double-sided option in the materials to get a decent output. But somehow, this does not work at all for some parts. Please see attached images. Does anybody have an idea why this is happening?
Scene file and images attached.

[hermit.crab]

This seems to be a geometry related issue. Try adding a Normal modifier and flip/unify the object's normals. You can also try adding an Edit Normals modifier and use the Reset option.

[kosso_olli]

This seems to be a geometry related issue. Try adding a Normal modifier and flip/unify the object's normals. You can also try adding an Edit Normals modifier and use the Reset option.

Well, I know about all of these options. However, there are a few downsides to this:
The normal modifier is an invalid option on CAD data. It will instantly break normals, resulting in faceted geometry. The same is true for the reset function of the Edit Normals modifier. This kind of makes sense once you understood how normals are working on imported CAD data. Please consider actually testing your advice before giving it to other people, no offense.

There are ways to fix the flipped normals using the provided tools inside the Edit Mesh or Edit Poly, so flipping is no problem. However, as this error is present on every part in the scene (about 7000 car parts), manually flipping by hand is out of question.

[^Lele^]

Could it be overlapping faces, Olli?
It doesn't really look like z-fighting, and should work anyways if it was, if dirtily, so it's a bit of a lunge, but maybe raising the ray offset slightly would work?
Regardless, would you mail me one such part?

[kosso_olli]

Could it be overlapping faces, Olli?
It doesn't really look like z-fighting, and should work anyways if it was, if dirtily, so it's a bit of a lunge, but maybe raising the ray offset slightly would work?
Regardless, would you mail me one such part?

There is a zip file attached in the first post. It is easily missed, but it is there. Feel free to tinker around, I didn't find a solution other than asking the client for the original Nurbs data.

[Vizioen]

Looking in clay mode at the geometry reveals that there's a lot wrong with it (also with edit normals it's apparent) Flipped normals etc. So I suggest your client makes sure the nurb model is fixed before importing.

edit: I should've read the entire opening post. My bad. There are some scripts that can flip normals "automatically" according to pivot points and whatnot, maybe that can be of some help, check out sini software.

There's a difference though between twosided and not 2 sided. If you put on a editable poly or vertex weld it kind of fixed things but also introduces other problems (smoothing)

[kosso_olli]

Putting an edit poly modifier on top of the delivered CAD files is not a good idea. This is an invalid operation which breaks normals, don't ask me why. Sometimes it is hard to see in the viewport, but if you look closely, you can see it. It is better to use right click->convert to edit poly.
The only thing I found out is this: On the upper part, the vertex normals are not perpendicular to the surface, but shifted or skewed to some degree. You can rotate the normals, but it seems to be a random arbitrary amount.

[^Lele^]

Yeah, geo is fubarred.
You did right asking the client for the source, imho: much better than a re-export (gosh.).