This is our method too. That said, it’s not that horrible of a job to just do it by hand. Put some thought into efficiency-- have all of your supplies in a cart that you can easily pull down the fenceline as your work, and keep your paint bucket at a comfortable height so you don’t have to bend down every time you need to refill your brush. And as a side bonus, your arms will get super muscular! 
No matter what, pressure wash them first, for sure, or all that work (and $$$) painting will just have to be repeated in a year or two when it starts flaking off.
Tip: When painting, it’s easy to get in the habit of trying to get all the paint off your brush before going back to your paint can / tray to reload the brush. Dry-brushing like that is a lot of wasted effort, so try to remind yourself to refill your brush often, so you’re not going over the same spot with the brush more than twice. 1st pass apply, 2nd pass smooth, then move on.