I recently bought a job lot of cast iron cookware at an auction to get the camp oven (Metters 14") that was part of the lot. Included was three cast iron kettles and two oval boilers all in good nick on the outside but heavily rusted on the inside. I've cleaned them up and painted the outside with oven black but want to stop them rusting out from the inside without making them useless to ever boil water in again, any ideas ? poddy dodger.

Sounds like a pretty good purchase there PD.
I have found that dilute hydrochloric acid is a good rust/scale remover for the inside of cast iron cookware. The idea came from its use in descaling old tractor engine blocks. Once you have them cleaned up pretty well, keeping them dry is the best way to prevent further corrosion. Perhaps silica gel crystals could help.

The old fountain I have has a coating of white calcium inside (lot of lime in Gippsland water) which has kept it in good order. Couple of other old boilers have surfase rust, but I dont bother about it.

Did you use a "paint on" stove black or the paste type?
cheers  mikel

G'day Mikel,
The three kettles and two boilers I cleaned up using electrolysis and a wire brush, I have just given them a couple of coats of "White Knight" oven black heat proof paint using a brush, I was'nt aware of a paste. On the outside they are all in very good condition and I think they might have been protected with some sort of paint/paste at one time, the makers name is clearly visible both on the sides and underneath but inside is another story, like you suggest I'll just keep them dry.
The camp oven also came up well, inside is as smooth as, the outside has minor pitting but I'm not worried about that, I have seasoned it with olive oil  as I do all my pots.
pd

PD . the stuff we used to use on the kitchen stove and oven was in a tin, wiped on with a rag then sort of polished up. We just called it "black lead" but would have had a brand name. This was about a 2 weekly ritual, mum liked to keep the kitchen looking "decent" :-?
I think the old stove was a "lighthouse" manufacture. Bit dim on it now though. :-/
cheers  mikel