Rust is a vector for more rust. You want to remove it before it eats its way too deep into the metal.
I’ve seen a video of the same stacking technique using birchtbark instead of leather. The effect was gorgeous.
Ah, how about I not be lazy and find the link… https://www.youtube.com/watch?v=BLnqr6IGVgs Maybe it was another one I was thinking of, but this short movie shows it.