BC 59 If that rubber isn't any tougher than that,probably need a better balancer!!
The deal with banging on the outside of the balancer has little or nothing to do with damaging the rubber, but more about upsetting the even tension of the rivets.
If one strikes the balancer outside of the hub, it can squarsh the rubber on one side causing a horrible wobble which can only be fixed by drilling out all of the rivets and replace them with 10 mm bolts.
I recently repaired one with that problem and was shocked at the soundness of the 70 year old rubber. It looked like new. It was not damaged by whatever hit it.
What happens is, the rubber is compressed but cannot return to its normal shape until the balancer is disassembled due to how tight the rivets are in their bore.