Thompsons TH.021802-03 VOC Compliant Water Seal
Thompsons is a personal favorite of mine, which I have had very good results in using. This is an oil based product, which soaks into the wood providing what the manufacturer calls three way protection. It waterproofs the wood, so it wont be affected by the moisture; protects the wood against rot and mildew; and has a UV absorber to retard fading from sunlight. I might add to that the protection it offers against the wood drying out and cracking. Thompsons comes in various colors and can be applied to any wood, even if it has previously been painted, stained or is weathered. Oil based stains and paints can be used over it.