Quite simple really. I keep animals for my own consumption and occasionally rear a few geese for that purpose. As little goslings are vulnerable, I start by rearing them in captivity, but with access to a small outdoor run. However, they do like to splash in water, even as youngsters,
so I gave them a heavily-rusted sheep-feeding trough to play around in inside the mini-shed. Unfortunately, all they did was dung in it. I'd clean it out, and they'd do the same again. Eventually I got fed up of washing it out as they weren't using it for play anyway and gave them a different splash area outside. Eventually the trough was filled with quite viscous pooh water/sludge. I moved them into a different shed for night-time confinement as they grew and abandoned the smaller space, including the now Cumquats trough. Some months later I cleaned out the mini-shed and trough and as I hosed the trough out, I noticed the bottom of it was grey. Confused, I looked more closely and it was pure grey steel, right down to the bottom of the pits on the surface, some of which were quite deep. Once dry, I coated it in Hammerite and the rust didn't come back and it got used as a feed trough again.
Now, I'm not recommending one gets geese to dung all over the 1920 Alfa Romeo you might be restoring to neutralise any rust it may have, but I am assuming there is enough acid in goose pooh to eat rust, so it could be a way of de-rusting a metal feeding trough for nothing. I haven't tried chicken, horse, donkey or turkey pooh yet, despite keeping said animals, but I se no reason why it wouldn't work. I do however use dried 'donkey pellets' as fuel for my multi-fuel Rayburn to heat water in the summer. We are in the middle of a cost of living crisis after all!
