Perhaps you ought to take a poll on whether spanking is generally painful, I think you might be surprised by the consensus of your peers.
Spanking is no longer spanking when it inflicts pain.
Straight from wikipedia.
There is no punishment mechanism to spanking without pain. This sort of strange homeopathic idea of spanking is unrealistic and I'm unsure why you're trying to paint this false picture of the world - the vast majority of parents who spank do so with the intention of causing pain to discourage behaviour.