Many moons ago we used a cut down coffee can, some wire and roofing tar. It worked very well.
You know, I was thinking that. Like Henry's 208.
I used to restore older motorcycles. If I could polish the chrome fenders up to an acceptable level, I'd then just coat the underside with that or similar. Perfect, lasts forever. Also roofs, etc.
I love 3M 5200, but it's messy to work with and in this app, I'd like something that spread and gave me surface sealing and rust-protection.