Painting weights can be challenging and one of the most annoying things to skin is meshes that have some thickness to it. The main challenge is to keep both sides of the mesh to deform exactly the same.
Following these 3 easy steps and make this process super easy.
In this video I use a custom script. If you are interested in getting this script and a small collection of scripts that I use when I rig, you can follow this link:
https://bit.ly/3G5LtMV