To effectively protect the wood, it is ideal to treat the entire surface before installation. However, if the wood is already installed and has turned gray over time, you will need to start by cleaning it with a Wood Cleaner & Brightener using a pressure washer. Once the surface is clean, sand it using a 100-grit abrasive and apply two coats of Osmo Decking-Oil. Be sure to renew the protection when the finish begins to fade. For further information, follow the instructions on the Technical Data Sheet of this product, available online.