All decks including new ones need to be cleaned before they are sealed. There is often saw dust, chalk marks, debris from construction, bird droppings, twigs and other environmental fallout that needs to be removed before the deck is sealed. Decks that have turned silver or gray in color need to be restored back to their natural color before seal is applied. Mold and mildew should also be cleaned from the wood before sealing. Stamp marks however cannot be removed by power washing.