Searching for pumice for a mechanic's soap using Bamagirl's recipe. Never having used pumice before I hesitate to order without more info. I've found several but the recipe calls for fine and I wonder if that is 4F or 3F? 4F to me seems to be extra fine and I haven't found a 3F designation anywhere. I intend to distribute this to a few bicycle mechanics I know to see how they like it.