Our club was given 7 grocery bags of 25 year old Panamic shells recently.
They had all been dipped in shelac.  Full strength bleach did the trick,
almost.  The ones that didn't come completely clean were picked with dental
picks.  Our shells, which were donated, had all been found in Panama ,
dipped in  that stuff, wrapped up in Miami Herald newspapers from 1964, and
obviously put away and forgotten.  But bleach helped considerably and I
changed the bleach a couple of times to renew to bleach's strength.
 
Good luck!
 
Carol