Slicer not slicing properly  

Dark green areas usually mean the model is broken in some way. If you are on windows you can usually use the right click and repair through netfabb option to fix them. If not then you can try some of the other software that allows repair of models, like mesh mixer or blender etc. 
If bits are missing then it’s certain that it’s broken. 

Slicer not slicing properly

Thanks for the info.  Before I sent this message I tried Meshmixer (that's where this object was created) but that didn't help then I tried Netfabb and that is very intimidating but looks like I have some thin areas (which I don't know how to fix).  I also tried detect thin areas but that didn't make a difference. FYI I'm running Windows 10 and Prusa Slicer 2.3.0.  I have also included a .zip file with a .emf file in it.

Right Rear Body 6

Your model is non manifold.  Basically a model has to be 'water tight' ie no holes in the mesh for it to be sliced properly.  That model has 184 non manifold edges.  It also has 719 intersecting faces which sometimes also cause the slicer problems but its the non manifold issues that kill it.  Also even if it was to be made manifold there are several areas that are too thin to print and it appears that the inside surface has been moved through another surface (mathematically turning part of it inside out, so sort of like an escher bottle) which is what I think is causing most of the intersecting face warnings when I run a scan on it.  Those inside out areas are the dark green section you see in PS.

Basically I don't think ANY automated repair tool is going to be able to fix this one.  I don't know where the original model is from but you may want to examine your source for errors.  Can I ask if you modeled it ?  or did you just try modifying it ?  you did mention mesh mixer which is generally used to modify meshes rather than create them from scratch.  The original might be workable but the model in the 3mf is beyond help I think.  

Unless a real hotshot cad designer comes along 🙂

Slicer not slicing properly

Thanks for the info.  It is very helpful. I also tried right clicking on the object and clicking on fix using Netfabb and my 2 shades of green turned into the 1 shade I normally get and then sliced the part and it was 95% better but still not there. I did that 4 times and it got a little better each time for first 3 times but then stopped getting better.

If your interested in the long story here it goes.

I bought a rendering of a 57 Vette done in 3dsmax. The stl that is created in 3dsMAX is just a zero thickness shell.  After trying many product I found that Meshmixer can extrude a shell.  So I did that on the whole car (extruded it 3mm). I then seperated the car into 6 shells (2 doors, hood, trunk and conv top compartment and the main body).  The car is going to be 1/6 scale (about 800 x 300 mm). I then printed the 5 pieces that fit on my MK2S and MK3S without a problem.  I then "cut plane" the body into 6 pieces and that's when these  problems started. Apparently when Meshmixer extrudes a curved surface it makes most of it 3mm but some of the areas are thinner. It also seems to extrude it on the inside (which is what I want) but I don't know what controls that.  Meshmixer is a free powerful program but there is very little documentation.