Light refracts in the core and is surrounded by cladding which acts like the walls of a tube the reflect wavelengths back into the core glass (some arguably call this core/cladding barrier the mode field). The only way light escapes is if the rated bend radius is exceeded, the light departs its core and a percentage is lost directly through the cladding material. The part that gets degraded over time with moisture is called acrylite. It only serves as a color coating fiber identifier and needs to be stripped during splice prep. It's why we can strip back acrylite and then seal exposed cladding in a clear heat shrink after fusion splicing with no ill effects. Heat, humidity, moisture over time will make the glass brittle though. Making splicing very old fiber a pain in the butt sometimes. No offense meant in correction, I just get meticulous about fiber stuff
Edit: I realize from your posting you've probably been doing this a very long time.