Armature is deforming too much mesh on one side. Rotating hand also rotates foot

Question

I am fairly new to Blender. I modelled a character using separate meshes for each body part (head, two arms, two legs). All of my child body parts are parented to one Body mesh.

I created an armature, and the rigged my mesh by selected my entire body hierarchy, selecting the armature last, and then clicking Parent -> Armature Deform -> With Automatic Weights.

If I go into pose mode, everything works fine on the left side of the character, but when I rotate bones on the right side, the mesh deforms incorrectly. For example, if I try to rotate the right arm, the right foot also moves.

I have applied all transforms to my mesh before parenting.

At first I was using Symmetrize for my armature, but when it didn't work, I created all bones manually for both sides. I had the same results in both cases.

I tried joining my meshes into a single body mesh before parenting to the armature, but this did not fix the issue.

None of my meshes have any modifiers on them before applying the deform.

All of my meshes and bones have unique names.

When I was modelling the character, I modelled the left arm and leg first. I then duplicated each mesh, and rotated them back into place on the other side. I am assuming this is where I went wrong, but I am not sure how to fix it.

Any help would be appreciated!

Answer 1

Sounds like Automatic Weight Paint failed.
Possible reason can be wrong Normal Face orientation.

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In this example I also model separate parts of body. Later I joined right parts together. When I Mirror right side to left side in edit mode with Ctrl+M+X Normals switched ... hmm. Automatic Weight Paint issue followed.

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To fix it check first normal face orientation, like enable Display Face Normals in edit mode. On left side is direction pointed inward. (Or you can display Face Orientation in object mode, but since this view mode use red/blue colors same as weight paint, I didnt wanted to mess up screenshots with similar colors here.)

Recalculate Normals Shift+N

Go to object Data Properties > Vertex Groups and Delete all Unlocked, and run Ctrl+P Parent > Armature Deform > With Automatic Weights again.

Note: I see red/blue color of face orientation vs WeightPaint Red/Blue is probably confusing here :)