The salt air of places like Martha's Vineyard is a killer to bikes that spend their life in it.

I've treated several frames with AMS Oil Heavy Duty Metal Protector. One $8 can will do 3 bikes.

For bikes that are going to see a harsh environment, I pack the bearings in lots of marine trailer hub grease; the stuff that can both hold up at highway speeds under load and then get submerged several feet to launch the boat. Nothing a bicycle see will ever touch it. (Any auto parts store.) I also use that grease on threads.

If you really want a finish that will protect a steel frame in any conditions. paint it with a two part epoxy paint. (A brush does a surprisingly good job. Epoxy "flows" as it sets up and brush marks disappear. (Don't even think about touching the paint at this time!)

Ben