It is definitely worth sanding a deck to ensure the wood absorbs new stain. Sanding a deck removes the outer layers of aging wood, allowing the stain to penetrate. It also gives a fresh look to the entire surface as part of the deck refinishing process. Without sanding, your stain or sealant is less likely to absorb into the wood. While sanding a deck is a time-intensive project, the rewards are noticeable and long-lasting.

The best way to protect a deck is to prevent damage from UV light, water, mold, mildew, insects, and abrasive cleaning methods. Sweep regularly to prevent debris that hides bugs and traps moisture that can result in mold or mildew growth. Use a deck cleaner once a year to remove algae, mold, and mildew. Avoid using pressure washers on high, as they can damage the wood. Applying a deck sealant can protect the wood from UV and water damage, and adding shade can also reduce sunlight damage.

Applying too much sealer to a deck might seem like extra protection, but it can cause more harm than good. Wood is a porous material with a limited capacity to absorb liquids. When you go overboard with sealer, the excess can't soak in and instead forms a film on the surface. This leftover layer is prone to peeling, cracking, and trapping moisture, ultimately defeating the purpose of sealing in the first place. 

There are more pros than cons when staining a deck. Deck stain is more affordable than paint, it protects the wood and allows the natural textures to shine through, and it can revive the appearance of your home. However, deck stain also requires prep work (sometimes days or more), carries more upkeep than a painted deck, and can fade over time if not cared for.