I have all the naming right (forearm.L / forearm.R etc.), now how can I mirror the weight painting to the opposite side so it applies to the opposite bone? I've only seen manual solitions (mirror copying vertex data and pasting it to the other bone and such). Is there a way I could do this automatically? I don't think this would be too hard to implement into blender, so I dont see a reason why this wouldn't be a thing already
Maybe you are talking about this method but I don't know if there's another one. Let say you want to mirror the left arm group to the right.
- If the right group already exists (because you've parented With Automatic Weight), delete it.
- Select the left group and copy it.
- Now mirror that copy.
- Give it the correct name, i.e. with the correct suffix. This way the bone that is supposed to control it will automatically do.