We hit lumber like that with a hand held brush sander before planing. It gets off most off the dirt and grime. Be sure to check every inch for metal.
Seems like it might be most valuable as fence boards again like Tom said. Unless it's something special and there is nothing else you could be doing then go for it. It just seems like a lot of time invested for meager gains. What's that old saying? Polish a t*** and polish a t*** and what do you have.....a t***.