Weird line between mesh connections I'm new here, but this thing is annoying me for at least a whole month. I've searched everywhere and didn't find anything.

If I join 2 body parts, for example the arm and the torso of my character, there will be a kind of line between both, and this happens not only on Blender but on game engines like Unity too, even if the material and the texture are the same.

How can I fix this?

To show an example of this, I split the arm off my character and re-joined it, and there is a division where the mesh was divided. The effect is visible.

Before:


After:
It seems that it is due to the flipped surface. So try to go to the edit mode with face selection mode, then select the face and then click on "Flip Direction" option as shown in screenshot below.