Here is the deal buy an old CI skillet, dutch oven pre 50's and you will get a smooth glass like finish. That's how they made them back then. I have well over 50 old pieces of cast iron dating from the late 1800's - 1950's and they are all smooth, like glass. I wouldn't waste my time grinding a cast iron piece to smooth it out not when you can find them in thrift stores, garage sales, craigslist etc..... under $20.00
PS- you do not have to pay the high price of an old Griswold or Wagner to get good quality. With those you pay for the name and don't get me wrong they are great quality CI but you can get lesser known names that were made just as well for pennies on the dollar. I have several Griswold and Wagner from the early to mid 1900's but I have several other brands that are just as good quality.