Best rust solution?

[Brian]

Pesonally, I don't like using acids of any kind on cast iron. Tends to eat out the carbon content- which is the pitting that has been observed with molassis treatments etc.Much prefer to use an alkali. As long as there is no aluminum in your cooling system [read heads], fill the system with caustic soda and drive the thing. The hotter it gets, the better the caustic works. Drain and refill as required, finally flushing with straight water. This will give you nice clean water jackets etc, improving heat transfer etc. Then, irrespective of where you live, tropics to the polar regions fill system with 50/50 mix of antifreeze and be happy.