Old paint usually has lots of grime, get the paint clean with mild soap and soft clot.
Rust spots need to be neutralized, if theyre mild you can try CLR (calcium lime rust) bathroom cleaner and a scotchbrite on the rusted areas. See attached pic.
Heavier, scaly rust will need some careful wire brushing to not remove paint, maybe some sanding, work slow and save as much paint as possible.
Protecting it after that, couple choices. Main thing is to get a layer of something on it to prevent air/oxygen/ additional rust.
Shoot some matching paint on it and wipe with thinner to blend it in, it will still look weathered but you need to get enough paint on it to seal it, need a good paint match first for this to look right.
Heavy coat of wax, I like paste waxes for this, particurly johnsons paste wax. This is the easiest and simplest method to me.
Oil coat, works well easy to reapply, I always seem to get oil on me later, don't care for this personally.
Clear coat - unless you kill all the corrosion, the clear will fail, just depends on how long. Then you're stuck sanding and re clearing or losing the original finish. Rattle can clear is not very tough, automotive clear is stronger and can be flattened, depends on how familiar you are with auto paint. I can assist if someone is interested.