I have to admit using cast iron for heating water is something I don't do.
I use them for food, things that are food from stew to chili and baked beans for larger pots.
My fav is a frying pan, a Wagner 10" with lid.
Now, these uses require seasoning to keep the iron in shape, make the utensil last and all.
This means getting ALL of the rust OFF. Then covering with thin coat of oil, very thin. Go over all surfaces very lightly with a dry towel to get rid of excess.
Put it oven over 400'F for 1 hour at that heat
Cool in oven and Repeat oil coating 2x more.
You should have a nice shiny hard coat and black.