My finest folder (Zeiss Ikon with Tessar T* lens) was totally gummed with "tar" when I got it. It seems it had been on display only in the home of a pipe smoker. Looking through the lens it was brown! The shutter was sluggish, the leather sticky and so on.
I took it with me to work in the North Sea and cleaned everything with alcohol, ether, trichloroethane and acetone. Now it's like new - totally pristine. Very cheap at about $35.-!