Once a wooden out-door item has been painted, you HAVE to keep the paint finish 'sound', or the wood will rot! Don't paint it while it is damp either (same reason). Moisture under paint causes rot!
Creosote (or a good substitute) is better, but most 'wood preservative' treatments are going to help somewhat. Creosote will still need 'alternative accommodation' provided for a day or two.

__________________
Flowers come in too many colours to see the world in black-and-white.