i use the same stuff on my fences as i do on my decking, its called liberon, i use the clear and it repeals water.
The stuff isnt cheap but its the best stuff ive found, with a lot of the cheaper stuff i found they pealed after a few years making a right mess.
I usually apply a fresh coat to the lot annually but this year i found it was still waterproof (probally because of the mild winter) so only applied in a few places.