Here is a 'robot' that has a chest, waist, and mid section. It's fairly simple, but it does for exposition:
I'll explain the loop cuts in the middle section below. The top cube serves as the 'chest. The middle cube as the 'mid section' and the bottom cube as the 'waist'.
I added an armature. It's very simple and the parent hierarchy is also simple. The ...
You can use a Vertex Weight Mix modifier.
Say you have Vertex Group A :
And B :
In order to subtract B from A, add a Vertex Weight Mix set to Subtract and to effect only group A.
Result when viewing the A Group after adding the modifier :
If weight painting a vertex group in a mesh differently is not affecting how an armature deforms it, one of the following is probably true.
The vertex group has no bone with the same name to match with.
The armature modifier is disabled.
The armature modifier is set for “Bone envelopes” instead of “Vertex groups.”
Check for these. If not, make sure you ...
Try checking if your mesh is actually a mesh. I had this problem earlier when I tried to add an amature to a model. The problem was the model was made from curves, when I tried to set the parent with automatic weights the option was missing. When I converted the model to a mesh and did the same process, the option came up. Hope this helps lol
You need to :
Go in Edit mode, some normals are flipped, recalculate them
Add some edge loops so that your armature is able to bend the object:
Reparent With Automatic Weight
Bring some corrections in Weight Paint mode
This model is little bit low poly for rigging. Try to subdivide it more, if that doesn't work then it has issue with the bone weights. Try separating the arms(as a different mesh) and then rig it with the same skeleton.
See if that works.
Here are the principles of bone control over vertex groups:
Let's say that I have a bone called Bone1 and a vertex group called Bone1 as well. The bone will control the vertex group that has its name (if it has been parented with an Armature Deform option). Here I assign a vertex group weight of 1 to the 2 vertices on the left, and a weight of 0.1 to the 2 ...
There is a data transfer modifier, which can transfer the weights between meshes. Depending on the meshes you might need to go in and change some things manually afterwards, but it sure takes care of most of it.
I would go with the Nearest Vertex sampling method. Topology ...