I've used Rust-Oleums "Rusty Metal" primer, it has fish oil mixed into it and is extremely effective at stopping rust, just like advertised, I have sprayed it on rusty metal and for years it stayed and did NOT let the rust spread.
They warn though, there has to be some rust to use this primer on, if you use it on clean metal the fish oil will come to the surface and cause any topcoat over it to peel away.
The rusty metal primer is a flat brown color, almost rust color ironically.
I have used 3M rubberized undercoating, that advertises it has no tar in it. Its expensive, but it looks great and it works great. i.e. cheapo undercoating that is mostly tar is like $4 a spray can, the 3M high quality stuff is like $10 a spray can.
One other tip I have never tried, but I have heard it works great, spray "Bar & Chain" oil all over the underside prior to winter. It designed for chainsaws (bare metal exposed to the outdoor elements), its thick and sticks to the surface, it has additives in it to protect against the elements and fight rust on metal. You may need a special spray gun and air compressor, or you could always paint it on with a brush.
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