FWIW, fish oil is what Rustoleum uses in their paint as an additive. As has already been said on fishing boats it was noticed that anything that got fish oil on it even in a salt water environment would never rust. So there is truth to what was posted about fish oil retarding rust. Commercial bridge paints also use fish oil because of the road salt the iron is exposed to every winter.
I assume this is why Rustoleum remains fairly pliable even after it dries it never get truly hard.
A newer spray I have heard a lot about is Fluid Film, last year I talked to a friend of mine in Erie PA, my former home town that owns his own garage. He told me most everyone in Erie has been using it in the fall to undercoat their vehicles, seems it has done very good for the users. I guess it's available in gallons that you can spray on with an undercoating type sprayer. Amazon sells it for $38.35 a gallon, $160.99 for 5 gallons:
https://www.amazon.com/Fluid-Film-Pr.../dp/B004NDDCKK
The major component is wool wax. I've used the spray cans and it does work on rusty stuff to free it up and keep it from rusting after going back into service.