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Get all the loose rust off with a wire wheel then put on some Jelonite rust preventer then paint with something
https://www.jenolite.net
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The modern jenolite stuff is shite.
https://www.bilthamber.com/corrosion-protection-and-rust-treatments/hydrate-80 is the one to use.
Or cold galvanizing spray, Had a 70's lancia shell (post fiat purchase series 3 and fiat metal) blasted and painted with this stuff and was left to the elements with a tarp over it for five years. The shell was still perfect.
After quite a few productive weekends, this weekend I only managed to remove the front bumper. Some seriously corroded bolts but glad things aren't too bad, just looks like some grinding then Hammerite for peace of mind. Might relocate the number plate to the side whilst filling the ugly holes that will be revealed too.