In The Swiss Family Robinson, what good uses did they put the shark's skin to?

In this book, the family takes all sorts of things that
they get from nature and puts them to good use.  For example, at one point, they kill a
porcupine and they end up using its quills for needles and for the tips of arrows, among
other things.  The shark's skin does not get used for anything so vital (at least not
that we are told of).

In Chapter 2, Fritz shoots a shark. 
In Chapter 3, they come across the dead shark and the father tells them to get some of
the skin because it can be used like sandpaper or a file.  However, we never see them
actually use it for that purpose.  Instead, the only time we see them use the skin is
when they wrap it around their legs to help them climb trees more easily in Chapter
7.