Oh! I didn't know you were cleaning those...
Next time, try Electrolysis.
Suspend the part in a tub of water and 1/4 cup Arm & Hammer washing soda.
Hook the + of a battery charger to some clean scrap iron suspended in the solution.
Hook the - of the battery charger to the part.
Turn on the charger, and walk away.
Come back later and it will be clean, gray, cast iron, ready for header paint.
Quick and dirty video:
https://www.youtube.com/watch?v=CqTID6rQ5JA