I remember from many years ago helping a friend up in Virginia make a leather bellows, and he showed me some different types of thin and very flexible leather. He had me to hold them up to the light, and you could really tell the difference between the density and size of the pores in some of the leathers.
He selected a piece that, although it had more pores per sq. in. that many of the others, they were much smaller and he said they would be easier to seal. He rubbed the leather with something that had black dye added which treated the leather, sealed the pores, and dyed it, all in one operation, and it had no effect on the feel or flexibility of the leather. He said that over time the leather might begin to leak light as it wears on the corners and folds in the bellows, but it will never leak through the pores as they were permanently sealed.
I have some really nice chamois (Kodak Photo Chamois) that I want to use, but need to get some of that leather sealing & dying stuff that leather working folks use. I don't know what it is called and where to get it. Can anyone help?