So I purchased a model and rigify refuses to work when parenting the mesh to the meta rig. I think it's because the arm clips into the armpit creating a vertex hell. So I made a quick arm rig to rotate the arms into an A pose, but this is my first time weight painting manually, and it's deforming like hot garbage. Any ideas on what I am doing wrong? I have attached the blend file (I think?)
You need to smooth out the weights between the shoulder and arm. The blur brush in weight paint mode can work quite well for this for this.
The Blur brush is a separate tool from the standard brushes and can be found in the left hand side tool menu.
You may also need to add a root/body bone and weight the stationary vertices to that, then blend between them. Turning on auto-normalise can help when blurring between multiple existing groups, as it will automatically adjust the weights of the others.
Additionally, check to see if your armature modifier is using vertex weights or bone envelopes. Don't envelopes.