Will 2f and 3f work for a prime absolutely they will. If you live in a very humid area such as the South East that’s what you may need. Then there’s some of us that are competitive shooters, we’re always looking for the edge. 4f is a minimum. Many of us use a kitchen utensil, a marble mortar and pestle usually about $20. Can easily and safely grind 4f to 5 or 6. Years back 7f used to be available but seems to have dried up. A problem did arise with 7f, in a wind the 7f would literally blow away. Using 6or 7 the pan and liner stays very clean. No need to brush.
Doc,