When I am cleaning the expandable watch bands, I remove them from
the watch and link the ends together with a bit of wire, I then
stretch them around an old coffee tin and then put them in the
ultrasonic. This opens out the links and lets the dirt flow out.
This has to be one of the most unpleasant jobs in the business,
although the state of the backs of some watches that come in for
battery replacement isn't too far behind. Can't the owners see this
Sod's Law, the worst one will come in as a rush job, as your having