Proper deck cleaning techniques
For cleaning decks properly, use a sodium hydroxide based cleaner that will remove any existing sealers, and to kill mold and mildew growth.
After applying the cleaner, let it dwell on the surface for about 15-20 minutes, then using less than 1,000 PSI, rinse it from the surface.
Then, thouroughly rinse the surface after and apply a wood brightner made with an oxalic acid base to balance the Ph of the wood, allow it to dry a few days then apply a penetrating wood sealer and stain.
Do not use a linseed oil based stain that is commonly found in Thompsons, Wolman, Behr or Olympic. Linseed oil is food for mildew! Use a product like Ready Seal or another comparable product that has a tint in the sealer for UV protection and oils that will moisturize the lumber.
These are basic wood restoration procedures used by professional wood restoration contractors across the nation. Please ask a pro in your area for services like this. Use this link: Pressure washing contractor directory to find pressure washing contractors in your area.