Although expensive for a large deck, drying oils like Tung oil, maintain the feel and texture of the wood while making it waterproof. The oil penetrates the wood fiber and, within a few days, completely hardens. It is usually applied by ragging with the excess wiped off. It only requires one coat. It will usually darken the wood slightly leaving a light honey-brown color behind. The only thing you need to do to prep the surface is to ensure the wood is clean and dry. You can lightly sand the surface if you want, but it’s usually not necessary unless the wood has something on it that you want to take off.