Used to spray around the post but decide to get in my steps and weed whack. Use blue plastic string and it does not eat into the post if careful. Timed my fence weed whacking yesterday it takes 7 seconds to do a post and walk to the next post. Have around 650 post in total so it is a few hours ever three weeks.
Use a Walker H27 mower with a 61" 3 blade deck that has a smooth round radius on the left side and can get up tight to the post and only leaves about 2' of long grass. If I keep up with the mowing the weed whacker works out fine. My land has a natural stream that feeds into the Vermillion river then Lake Erie and don't want the run off.
I don't like all of the plastic parts weed whacking creates, wish I could find a natural string product that would work.