The family members obtain the shark's skin fairly early on in the book. They are excited by the possibility of its being a valuable tool, and we are told that they believe the skin is rough enough to use as a file.
Let us try if we can induce these greedy birds to spare us a bit of the shark’s skin; it is extremely rough, and when dry may be used like a file.
Shark skin is rough if rubbed against the grain. A shark's skin doesn't contain scales like other fish. Instead, a shark's skin is made of tiny, hard, tooth-like structures called dermal denticles. They are asymmetrical, and feel smooth when moving your hand from nose to tail; however, they are rough like sandpaper when felt tail to nose. The family doesn't use the skin as a file like they initially hypothesize, but the family does adapt the skin in a way that aids them in tree climbing.
So saying I held up buskins of shark’s skin which I had previously prepared, and which I now bound on to their legs. Thus equipped they again attempted the ascent, and with a loop of rope passed round their body and the trunk of the tree, quickly reached the summit.