Hello,
I’m having a problem with a mesh seam I can not get fixed. I think it has to do with normals or shading.
Would be great if someone could help me making it seamless. And of course if would be good to know how to fix problems like this. I have attached the files. Thank you 
NeckSeam.rar (3.1 MB)
The standard initial diagnostic is to check for double verts and incorrect face orientation.
Also make sure there’s no internal face there.
There is a seam because the head and body are separate objects. Also, the mesh is a mess. Lots of unnecessary verts around the neck. You need to join the head object to the body and then spend some time cleaning up the geometry.
Alternatively, you could do a “transfer normals” but i agree the geometry approach is best.
I need both of them as seperate objects. It should not be noticable by the shading. Ok,I’ll give it a try
I am not sure if this works for you. Things I did:
I am not into character modeling, but I did what I usually do when I get these
NeckSeam.rar (1.2 MB)
issues.
When I join the objects the UVs get lost. Is there any way to keep them? Both of them as selection or something?
Looks promising, but sadly not a fix fot me. Needs to be triangles and to shade real smooth. But thanks for your effort.
Welcome. Maybe someone else can help you.
Ok, I fixed the Mesh (I think) but now I have no UVS any more and blender refuses to transfer them. What should I do? It’s no solution to have no textures at all 
How can I do this? It might be better because the other approach I lose all textures and Uvs
Here is a file with better shading. But how do I get the UV and texture back on the fixed objects?
For now it is like: Object with UV and textures having bad seams. Object with fixed seams having no UV nor textures. UV transfer dosn’t work because one of the obects has 5 faces less due to fixing.
So this is not a solution. In the end it is nothing at all 
P.S. I tried with DataTransfer but I have no idea what settings I need to pull that off. All setteings I’ve tried don’t do anything at all.
I think it’s very bad part of blender that when joining objects all UV information is wiped. Cinema4D can join objects and keep that. Sadly I can not do the mesh fixing in C4D because it messes up all kind of other stuff. Why, just why has blender to delete that.
Fells so goo to be stuck between a rock and a hard place.
BrokenUV.rar (3.3 MB)
As you can see no UV. Original objects are also in the file.
I just tried your mesh. Joining head and body part worked pretty fine and kept the uvs. Be aware that joining them does not mean that doubled verts merge. It just puts everything into one meshobject. Then I manually merged some of those vertices at the neck testwise and the neckseam disappeared. But this mesh is not in a good condition. It has separated surfaces everywhere with doubled verts. Merge by distance instantly removed 1200 verts. So clean it up first.
But how? When I join them they are gone. I did nothing else then joing them and fixing the neck, then seperating them again. But as you can see in my last .blend file there is no UV anymore. I tried again and found that as soon as I join to objects the UV Maps Tab is cleaned. Ending up like on the screenshot above: Empty. So how did you do that?
Well in Object mode I selected LOD_1_Group_0_Sub_3__Skin_Mat and LOD_1_Group_0_Sub_3__pl2001_Face_Mat
If I use Object->Join I still have DiffuseUV and LightmapUV.
Again, whats the version you are using?
I do the same and they are gone. Version is 2.8 
I’ll try with 3.4 but need to install that. What version do you have?
I tested with 3.5 alpha currently, but yeah I’d also suggest 3.4. Good luck.
It works now. What else can I do to make it more smoth? After joining objects and fixing all vertecies there is still something off with shading/normals. Just smothing them dosn’t make it real smoth.
Look into how you can use the data transfer modifier to affect the normals/shading.
