The varnishing of cells?
Where does this get a mention? It’s in the study notes..
http://basicbeekeeping.blogspot.co.uk/2011/12/lesson-113-sticky-subject-of-propolis.html
They asked lots of people too.
In Ribbands, Chapter 27, Huber (1814) observed that new combs become more yellow, more pliable stronger and heavier and sometimes there were reddish threads on the inner walls. Chemical tests showed this was propolis.
Source: Uses of propolis