I am lazy. I use electrolysis with a automotive battery charger, and a solution of washing soda and water.
Attach black lead to your submerged cast iron, and the red lead to a sumberged piece of scrap steel. DO NOT let those two touch.
Plug in the charger and let it 'cook' for a day or so and check the progress.
Works for me
Google the washing soda electrolysis approach for the recipe.