In most cases, sanding an old deck before staining it is necessary to achieve a decent finish. If your old deck has a relatively new stain finish on it and you're only doing touch-ups, you can generally skip the sanding part. However, if your goal is to make the deck look and function better, sanding first is a must. 

Pressure-treated decks last 15 to 20 years on average, but with proper maintenance, you could get 40 to 50 years of value from yours. Reseal your deck every one to two years, and replace old or rotted wood as soon as possible to avoid structural damage. Where you live plays a role in the lifespan of your deck. Intense sunlight, rainfall, wind, and hail can reduce the deck’s lifespan.

When staining a deck, don’t skip surface prep, avoid staining damp or dirty boards, and don’t apply stain in extreme heat, cold, or wet conditions. Never overload the wood with a stain, as too much can cause tackiness or peeling. Also, don’t forget to test a small area first to ensure the color and finish look right.

It is possible to pressure wash your deck, but it’s usually best to leave the work to a professional for two main reasons. First, power washers are expensive, and you can avoid high upfront costs by hiring a deck washing pro instead. Second, you must be careful to use the correct setting to avoid damaging your deck material. You can only clean soft woods like pine with a wide spray and a low pressure; otherwise, the water will leave unpleasant marks on your deck. Hard woods, composite material, and metal can stand up to increasingly higher water pressures.