Also the "Weight Paint" mode (I know it may seem wrong or completely not right)
It's hard to tell without the file. here couple of points to consider: Activate normalize in options
go through the vertex groups to verify that unrelated bones don't influence the given region. Ex. fingers on the upper arm. If yes, then subtract it.
In case you didn't try watching - enter link description here DanPro tutorials are brilliant and provide useful tips.
Remember also that the hand is a very complex mechanism, not only 3D, real hands are not intuitive organs, it's one of hardest parts of rigging.
Well, obviously you have to modify the weight painting a little. Simply selecting "automatic weights" is more than likely to give you unexpected results. I'd say subtract most of the weight paint and aim for the joints to be the most influenced (red). Keep tweaking until they stop having that weird bend to it
- Your bones are desegmented, bring them back to 1 segment:
- Put your Subdivision Surface modifier under the Armature modifier. Enable the Preserve Volume option of your Armature modifier:
- Recalculate the normals of your mesh (select all in Edit mode and AltN > Recalculate Outside:
- Add some edge loops around the joints (elbows) to have a smoother bend:
- Reparent With Automatic Weight: