This is an object made of two components joined together. The problem is that, despite the fact that they are joined, the boundary between where each originally was is still visible. One object is the lighter-gray part to the right and the other is the darker-gray part to the left.
To try to fix this I have split it into its original components and tried the following with them:
- Selected both objects. Joined both objects with command-j. Entered edit mode. Selected all vertices with a. Merged by distance, threshold 0.01 meters (i.e. enough to merge any directly overlapping vertices). Result: seam remained.
- Selected both objects. Entered edit mode without joining objects. Selected all vertices with a. Merged by distance, threshold 0.01 meters. Result: no vertices removed and seam remained.
- Selected both objects. Joined both objects with command-j. Entered edit mode. Selected all vertices with a. Merged by distance, threshold 0.01 meters. Opened normals menu via control-N. Tried flipping normals, recalculating the outside normals, recalculating the inside normals, merging the normals by faces, averaging the normals (by custom normals, face areas, or corner angles), and smoothing and resetting the vectors. Result: none of these worked, seam remained.
- Selected one of the objects. Added boolean modifier to it, subtype union. Eyedropper datablock selected as the other object. Both objects joined via command-j. Selected all vertices with a. Merged by distance, threshold 0.01 meters. Selected all vertices with a. Tried everything I did with the normals in (3). Result: nothing worked, seam remained.
- Same as (4) but with the other object getting the boolean modifier and the first object being selected with the eyedropper. Result: exactly the same, nothing worked, seam remained.
- Selected both objects. Used control-a to open whichever menu that opens; tried every option in it. Result: nothing worked, seam remained.
- Set up a boolean modifier as in (4) and (5), then tried every option in the control-a menu as in (6). Result: nothing worked, seam remained, with an interesting side effect that applying anything to do with the rotational data just resulted in a horrible fused mess.
- Selected both objects, converted both to a mesh with the "merge UVs" option on. Selected all vertices with a. Tried every single normal-editing option as in (3), then merged by distance, threshold 0.01 meters. Then, when this didn't work, I reverted it, merged by distance first, then tried out each option in the normals menu. Result: seam remained.
- Joined both objects, gone into the object data of the resulting single object, tried everything from auto-smooth to clearing any custom data they had. Result: seam remained.
I'm fairly certain this has something to do with either bad normals or vertex groups but I have no idea what nor any idea what to do about them, hence this post. It may also be good for question-answerers to know that each component is showing up as a different color in Sculpt Mode, which to me implies that while their vertices may be joined there's some different level on which they're still separate.
