Grinders influence the flavor of the brew from espresso machines, but not from the AeroPress. With the AeroPress, the effect of grind is only that finer grinds will deliver a stronger brew for your money. But you can also get that by using more coffee. When using fine grinds, you must press gently and allow more time. If you press hard, the bed of coffee will be compressed and block the flow. I like to press into a clear glass mug and press barely hard enough to make a slow trickle of coffee into the mug. Pressing takes about a minute.