There's a guy who frequents flea markets I go to (he sells cast iron stuff) and here's what he does: He mixes up a strong solution of lye and water, submerges the piece of cast iron in it and then lets it soak for several days. He says it takes off everything.
If you try this, use loads of safety precautions. I used to work with lye solutions in my early Air Force days when I was manufacturing liquid oxygen and we always wore a full length heavy rubber apron, full face shield and rubber gloves that went up to the elbow. No shorts or flip-flops -- wear long pants, a long-sleeve shirt and work-type, high-top leather shoes. Even after all these precautions, I was amazed that my fatigue uniforms would eventually end up with holes in them.
BIG SAFETY NOTE: DO NOT, I repeat, DO NOT ever pour water directly onto lye. It will result in a violent chemical reaction and blow everywhere.
Also, use cold water, cause this stuff really heats up once combined. Stir with wood ladle until it's all dissolved. Never use the ladle again for anything else but mixing up lye solutions. Also, read up on use directions, too, so you konw what type container to mix this stuff in.
Remember this little saying and you'll be safe when mixing a lye/water solution: "It always snows gently on the lake, but never lakes on the snow. Meaning: lye goes slowly into the water, and not water into lye.
Once you get your cast iron nice and clean, you can season it. The old timers used to build a big fire, let it come down to a pile of good red coals, then place the cast iron in it and cover it up. Leave it there until the fire goes cold. Oh yes, before putting the piece into the coals, they would coat it real well with rendered down Bear fat (stuff really seals up pores in old canvas tents, too, making them waterproof forever. At least it did when it was still available in the late 60's. Don't know about now, though.)
