I didn't try it, but shrink wrap or especially shrink tube should work if you put it around in bone dry conditions.
The shrink tube presses out the air if it contracts.
If you have enough time you also could ask these Austrian guys here what they recommend. I guess they have the most experience in this case.
Flints | Treibacher Industrie AG
Dr Carl Auer von Welsbach invented the flint in 1903. Today, Treibacher produces flints from Cereisen, also known as Original-Auermetall®.www.treibacher.com
I have a feeling it would actually hold the moisture increasing corrosion.Shrink tube won’t stop water ingress unless you use glue lined stuff but then you won’t get it off your forested easily.
I have used plenty of shrink wrap making automotive looms over the years
All metals and alloys corrode, it is just the rate that varies ...Do ferro rods really need protecting?