I think that all of the dirt and rocks running through the trommel was about like sandblasting and slowly wore away at the urethane.
No, it was falling out in long ropes.
Urethane calk, like rubber, is extremely resistant to sandblasting.
I am not sure why he designed it so it needed a sealant, but urethane would be an excellent choice in that application. That's what is puzzling.