As mentioned above, mercury causes massive structural problems once it comes in to contact with aircraft parts.
Biggest problem though is the extreme difficulty in tracing where it has come in to contact, and what parts are affected, and thus which parts are due to be repaired/replaced.
Apparently, this is exasperated by the fact that 'visually', there is no change to the metal, apart from, as mentioned above, the changes to its structure and how it copes (or not!) with stress loads.
Southern Air Transport had a B747Classic Freighter which they had to declare an insurance right off after a mercury spill (approx 1995/6). The aircraft subsequently re-entered service with ???? (Polar?) but this was subject to massive question marks as to whether it was fully repaired or not. Not sure what happened to it since...