Hello John,

Some rust in an iron chagama is inevitable.  If there is not a lot of rust you can probably get way with boiling water in it and making sure that it is dry (put back on the burner on low until all the moisture has evaporated).

Here is a quote on care from TeaToys.com

C. Metal and water
In Chanoyu, almost all metal is used in some connection with water: kettles, flower vases, kensui and futaoki, and sake servers. (Exceptions: hibashi, haisaji) Bronze and brass mizusashi also exist. Thus, most metal utensils are provided with a patina by their maker which protects the piece from the worst damage but with use, ths patina may become thin or dissapear and the alert Chajin must take good care of his/her utensils.

1. Iron kettles and tetsubin:
Once a kettle or tetsubin is cast, it is coated on the outside with a patina called ohaguro, "tooth-black" and the inside is coated with lacquer or some synthetic.
To "cure" a new iron kettle or tetsubin, fill it close to the brim with fresh, filtered water, especially if the water in your area is high in minerals. You'll have to have a source of such water anyway, since minerals change the taste of the tea. To this water you may add Japanese sake and/or green leaf tea, old matcha, even black tea as a last resort, and boil for several days or until the funny smell and taste goes away. You must boil the kettle for several hours at a time, replenishing it with pure water to keep the level topped up.
After this treatment be sure to do one treratment of just pure water lest you find a soup when you go to make koicha. (Yes I did this once too)
DO NOT BOIL DIRECTLY ON A GAS BURNER!!!
Rust is the oxidation of iron. Oxygen in the water vapor in the gas fire will rust the bottom of kettle just as fast, maybe faster than leaving it out in the rain. Trust me. Best case is boiling over sumi, charcoal but who has sumi to burn these days? I do not know if barbeque charcoal would be a good substitute. So if all you have is gas, put an iron plate or something in between so that the gas does not get near the kettle.
Some new kettles have a glass-like inner coating on them which is supposed to prevent rust. Nevertheless it is only a good cautionary practice to rinse and boil even these kettles once or twice before making tea in them for Guests.
Iron sake pourers, called kannabe or choshi, should be cured once before using. During the chaji, the kannabe should be heated with hot water just before the sake is poured in. When the kannabe comes back, the sake should be poured into something else and the vessel filled with hot water until needed again. After the chaji, try to dry the kannabe on sumi.
Avoid keeping kettle or kannabe near food being cooked. Any oil, or soup stock or any kind of splash will show up on the kettle's surface and "ruin" its appearance.
NEVER STORE IRON IN CLOTH OR PAPER!!! It absorbs water vapor and releases in the confines of the box, causing automatic catastrophe. No matter how it comes, unless you live in Death Valley, DO NOT STORE IRON WITH ANYTHING ABSORBENT IN THE BOX!

Hi Sweet Persimmon

sweetpersimmon wrote:

NEVER STORE IRON IN CLOTH OR PAPER!!! It absorbs water vapor and releases in the confines of the box, causing automatic catastrophe. No matter how it comes, unless you live in Death Valley, DO NOT STORE IRON WITH ANYTHING ABSORBENT IN THE BOX!

Why do you say this? Every kama, tetsubin, &c that I've bought has come with a saffron colored cloth to wrap it in inside its wooden box.