[limograf]

The Forth Bridge is a very long bridge that is permanently being painted. "Like painting the Forth Bridge" is a job that (feels like it) will never end.

https://idioms.thefreedictionary.com/be+like+painting+the+Fo...

[zentiggr]

Thank you for this - the most interesting comment of everything I've read in this thread

[sleavey]

I'm afraid this is an urban legend [1] (the painting, not the long bridge).

[1] https://en.wikipedia.org/wiki/Forth_Bridge#Maintenance