In alt.home.repair
"D Smyth" wrote:

A pressure washer does a great job, even taking off the remnants of the old
stain. The secret is to find just the right distance. Too close and you can
indeed damage the wood. I use my Karcher with the nozzle perhaps 5 inches
away. Longer distances are still very effective in blowing away leaves and
cobwebs and crud, especially in corners.
Just make sure the wood is dry again before you apply the coating.

I've pressure washed PT wood decks with no trouble. Rather than buy
deckwash, I mix my own combo of 2 scoops laundry detergent in a 5 gallon
bucket with about 1/2 a bottle of bleach. Spray it on softly and let it
soak 15 mins, then blast if off board by board. It looks brand new. And
yes, let it dry thoroughly before sealing.