I think it refers to the Forth Bridge, which, it is said, takes so long to paint that, once the painters have done a complete coat, it's time to begin repainting it again, so the job never ends.
Indeed. Unfortunately, to save money, they stopped painting it regularly, and huge rusty chunks started falling off it, particularly onto North Queensferry. (My dad lives near there and I've seen it myself). I try not to think of the massive bolts on the ground when crossing on the train..
I like to know how much of everything as well - but I still throw things into the oven at whatever temperature I think is best and cook until not quite black.