Some leathers should not be used on fountain pens....from my reading.
Especially in hot humid climes. Even rubber sac pens died early out there where the ED is still king.
I wish I could help you.....you have tried a light rub of Semichrome?
Though your problem sounds a bit deeper than what Semi-chrome can do.....Nova?...Novas? from the States...a 3 stage polishing system, might be of help. It can clear up jet plexiglass.
A long time ago I'd wanted some but they didn't want to ship to Germany.