This is how I did mine. I left the cast iron parts, (note 1) of my engine for about ten days in a plastic garden trug containing a caustic soda mixture, (note 2) ; this removed any grease, (with help from a pressure hose). Caustic soda doesn't affect clean iron. With a new CS solution I then wired them up to my battery charger, (Note 3) (-ve) with a (+ve) cathode, (note 4) in the juice, turning periodically; this removed rust. When the metal was rinsed and dry I sprayed them with a semi-gloss aerosol heatproof paint. Looking at old photos I think semi-gloss looks about right.
Note 1: Only cast iron; not aluminum
Note 2: Dangerous stuff but cheap and easily available in the UK. Add CS to water, never add water to CS. Wear rubber gloves and goggles when near to CS. Do not breath fumes.
http://chemistry.about.com/od/labrecipes/a/sodiumhydroxidesolutions.htmNote 3: I had an old unregulated battery charger that was obviously capable of pumping out real amps. Unfortunately I burnt it out later while attempting zinc plating.
Note 4: I think that's the correct polarity
