In MY shop, I'd knock-off any loose rust in the glass-bead cabinet, or with a wire brush, or whatever I found handy. Then, it'd get set-up in an elecrolyte solution, and have a battery charger hooked to it.
Electrolytic rust removal of cast iron will remove ZERO non-rusted material; you could leave it in the tank for weeks if you wanted to. The downside is that it doesn't work real well on interior surfaces, and that's a major part of where you want the rust gone-from.
You'd need a plastic tank big enough to contain the part you want to treat. A plastic 15- or 30-gallon drum works great. Small parts could go in a one-gallon plastic pail.
You fill it about 2/3 with water, and add
WASHING soda. This is
not the same as baking soda, and it's harder to find in the grocery store.
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You grab a sacrificial chunk of iron or steel. Something that can rust away to nothing and you don't care about it. Scrap sheetmetal works great, especially if you form it so it's wrapped-around the part you want to "save". Old exhaust manifolds work OK.
DON'T use stainless steel.
You need a battery charger--more powerful is better. I bought a 24-volt charger specifically so I could do electrolyte rust removal quicker than with a 12-volt charger. But don't get me wrong--a 12-volt charger works fine, just takes longer.
You bolt a STEEL strap to the part you want to clean, and a STEEL strap to the "sacrificial" iron/steel. Both parts go in the electrolyte solution--
close together but NOT TOUCHING. Hang 'em from the rim of the plastic barrel, or from a non-conductive (wood dowel or similar) laid across the top of the barrel.
Connect your battery charger leads to the steel straps that stick out of the electrolyte.
DO NOT SUBMERGE THE BATTERY CABLE CLAMPS!!!!!
Really, really important: The positive lead goes on the SACRIFICIAL item, the negative lead goes on the part you're trying to save. Do not screw this up.
Skim the scum off the top of the electrolyte now and then.
As the battery charger pours electrons into the parts, the rust jumps off the part you want cleaned, and corrodes the sacrificial iron/steel. Depending on the piece you use as a sacrificial element, you may need to disconnect power, remove the sacrificial piece, and sandblast it clean again. That restores the current flow and speeds-up the process.
And you'll almost certainly have to rearrange the parts--this process is most effective where the rusty part and the sacrificial part are closest together, so you'll have to move them around to get the whole part cleaned.
Your "cleaned" part comes out of the electrolyte with a black discoloration. Leave it, or wire-brush, sand, grit-blast the "black" off. You'd do less damage to the part to remove the rust with electrolysis than with heavy grit blasting, then remove the black with light grit blasting.
This does not work with aluminum, I don't think it works with brass, magnesium, copper, etc. IRON/STEEL ONLY, and
don't put stainless steel in the tank either.