Model parts stretching when moving armature

Question

I just made a human model and rigged it, but its body parts are stretching unnaturally when I'm moving these bones (images provided):

• upper arm
• forearm
• hand (palm)

Should I do retopology, weight repainting or something else? Thanks in advance The model on blend-exchange:

Answer 1

As you see you have inverted normals. To recalculate the normals, select all in Edit mode and press ShiftN. Now if you parent again it will work fine.

Answer 2

The issue is that your weights are not assigning the mesh parts to the right bones.

First, in object mode, select the armature object. In Properties Editor > Armature tab > Viewport Display enable "Names" so that we can see the bones names in the viewport:

Select the mesh object, ↹ Tab into Weight Paint mode.

You can Shift RMB a part of your mesh to see the list of vertex groups present where you clicked, and select one to paint:

If you check your upperArm or foreArm vertex groups, you will see that they barely affect the arms meshes:

These should be mostly yellow - orange or even red for the corresponding bones.

Before correcting that, make sure to enable auto normalize and multi-paint:

This is so that painting weights automatically "takes" the weights you add in one group from the others. This is an easy way to avoid issues.

Then, select the draw brush and paint the weight on the right bones. First paint the areas that should only be moved by that one bone in red, then use the smear brush to do the transition areas, and make fine adjustments with the draw brush by setting a different weight: