I've heard of using glue or nail polish..but both seem like they'd get sharp edges.

There is some stuff that is sort of fake skin (pharmacy/bandage section) and I've heard of people using that with good results. I think it would stay flexible.

When I thought I'd rather try a product than wait for natural toughness, I asked around and was told that quilters use iodine to toughen the tips of their fingers.

It doesn't take long to build callouses the old fashioned way..