My NOS Longines Seastar 7 has been sitting for about 30 years. Over that time the caseback O-ring has transmogrified into a ring of thick black sticky goo. I'll have to strip the watch to get rid of it as doing it in-situ would be suicide, this stuff stcks lick sh!t to a blanket. I've taken small samples and tried to find out which solvent will break it down but no success, any tips are welcomed. Have tried: Methalated Spirits Shellite Kerosene Mineral Turpentine
I use my ultrasonic tank (for bracelets and cases) with a diluted solution of Mr. Clean. It will dissolve this black goo entirely and does not affect the case or crystal at all. Alcohol will also work, and I use the 99% pure variety for many jobs in the shop. It will take more effort than using the ultrasonic tank though. Cheers, Al