On some cast iron you will find that the surface has been machined, such as with my Wagner chicken fryer. For me this is an undesirable trait, as it is hell to try and get a seasoning coat to stick to a machined surface. If that is what you want, then you can get one of those flexible automotive cylinder grinding wheels and chuck it up in your drill motor and have at it. The rough surface on cast iron cookware allows the seasoning coat to penetrate and adhere to the metal, creating a sitck resistant seal that protects the metal surface from oxidation and the food cooked in it from sticking too badly. Over time, many coatings of seasoning will build up, and this is highly desired by cast iron cooks. Of course, there can be too much of a good thing, and so the buildup will only get so deep before it begins ablating away, thus keeping the overall coating within reason. It is just carbon buildup with a little unvolatized organic material (neutral hydrocarbons) helping to bind it.

Without walking too far off the deep end here, I would recommend not smoothing the cooking surfaces of your cast iron. If you want a smooth surfaced dutch oven, go with one made from regular or anodized aluminum. The rough surface of cast iron cookware is deliberate.

The reason my fryer and my comal are smooth surfaces has to do with intended use. The fryer is used for nothing but deep frying with little chance of anything getting stuck, and the comal is used for nothing but making flour tortillas which cook relatively dry, and yes, they are both well seasoned inside and out, and what a pain that was.