The knife that slides up and cuts the 35 mm film in the Rondinax 35/35U and in the Essex 35 is often rusty or corroded. Very rarely were the ends of the tanks taken off to clean the knife or the inside of the compartment holding it and the thermometer. I have cleaned the knives by just rubbing them over with wet wet-and-dry emery paper. Sharpness doesn’t seem to be a problem because they work by sliding into a slot on the roof and really dig into the film in order to cut it. However, I just rubbed the edges with a small carborundum stone to restore the edge.
This photgraph shows the inside of the end compartment with the blade and spring re-attached.