I use a pump sprayer, cuprinol I think, and wood preserver, most important thing with fencing panels is to have a shamfered top, so water runs off and away, rather than down, and also so the panels aren't touching soil at the base. Spray the preserver on every few years, takes minutes per panel. I would also like to add, don't buy the rubbish panels from B&Q or Homebase. They charge 20+ per panel and the stuff is rubbish. I found a local place to me who made panels to order, 6x6' for less than 20 pound, they are vertically lapped, and set on 2x2" batons.. they weighed a tonne and were treated. Thinking I won't have to worry about rot for a longtime, and they were cheaper than the garden centres.