Power washing by itself will only blast off the dirt from the wood surface. Too much pressure will also raise the grain of the wood and cause splintering and severe damage. 80% of decks today are sealed or stained. Therefore, power washing alone is not recommended in most cases. Re-sealing the wood without stripping the old sealer will create a heavy build up of finishes that will prevent the wood to breath and the moisture to escape. This will lead to permanent wood damage such as cracking, splintering or warping. Your wood may also turn dark grey or black.