Sometimes it can be a symptom of gastric ulcers that can pop up if the horse has been under stress.
I see more wood chewing if the fences are new wood or wet and the horse has run out of hay. Slow feeding would help with ulcers as well as boredom.

I have used that cinnamon chew stop stuff to paint wood fences in the past, it is potent, enough to melt the plastic cup I was using to paint it on. I've also seen people have luck painting wood with used motor oil. Both things leave marks on the fence.

IMO nice hot Hotwire works best to preserve any type of fence.

"It's really quite amazing what a horse will do for you, if he only understands what you want. 
And it it's also quite amazing what a horse will do to you if he doesn't." ~ Bill Dorrance