I do not understand how to combine the power Immunity to Normal Weapons and any other armor the spirit may wear.

Are the whole thing added together, a bit like shooting through a barrier?

You mean if a spirit casts the spell armor on itself? So you mean if a force 6 spirit has it's immunity to natural weapons and force 6 armor on itself would they stack if shot at by a weapon? I think they do. I mean you can't really put physical armor on a spirit (unless the spirit is bound inside the armor or possessing the armor.) If you mean this situation then yes they stack. If you are wondering about the spirits "natural armor" armor and its "immunity to natural weapons" armor it is actually one and the same. Force 6 spirit - 12 'immunity natural weapons'/armor.

Ok, I just wanted to be certain.

A body 6 person can have an armor rating of 15/13 unencumbered, in non-milspec armor, post-Arsenal.

Adding a spirits forcex2 to that when I'm possessed just seemed, well unseemly, so I thought I'd ask here to be certain.

I'd better mention I roll 10 dice to summon and we play usually with heavy use of Mana Static, so its not as crazily overpowered as it could be.

Thanks for the reply.

Yeah possession is pretty nuts. The armor stacks, but remember the "immunity to natural weapons" only goes up to force x2. If you have 7 armor and the force 6 spirit possessing you, you have basically 19 armor but an immunity to natural weapons 12 <--- it's always going to be spirit force x2 no matter what other armor you have.

Does that mean you can roll armor to reduce damage before checking if it breaks immunity?

I think a very good idea is No, no it doesn't. Immunity to natural weapons is just a cap. If you break that cap then armor starts to apply after the fact.

"Logically", the armour is hit first and saps some of the round's energy, so if it knocks the remaining DV below the immunity level, it should spang off and do no harm. Not that this would be good for game balance...

I think this really cuts to the heart of the issue. Logically, it would work that way, but it will probably lead to a broken game. For the sake of your game it should probably be handled as a whole lot of armor dice, but still only that base amount of immunity, checked against the initial DV. As for the why of it: it's an abstract armor system in the first place, so best to just move along.