The best finish for an old deck is a wood stain that not only adds color but also conditions the wood to revive its cells and replace lost natural oils. Stains like Armstrong Clark are recommended for old decks due to their non-drying penetrating oils, which deeply penetrate the wood and rejuvenate its structure. These conditioning oils remain within the wood, while the drying oils create a protective layer on the surface, ensuring weather resistance. This combination of conditioning and weather protection is vital for reviving and preserving the cellular structure of an older wood deck.