Leon, first you want to know how old the deck is and if it's ever been power washed. Pressure washing removes the factory protective coat from the surface. There are at least two threads on the BB under "Deck Cleaning". Be sure to read through those, lots of good info. there.

As Bob said, if pressure washed in the past, there could be start and stop marks from the wand. Just let them know ahead of time. That being said, we softwash about 8-12 composite PVC deck a week. And it works great. We use anywhere from a house mix (1%) to a roof mix (3%) depending on how infested it is. Below are some pics of a ramp we did that was pretty bad.

Call or PM if you have any questions.

Thanks Chris. Yes we used only a softwash process. The ramp was hit with a 3% mix and let sit for about 15 minutes. The steps  with a 2% mix and a 5 minute dwell time. Basically we do the flat surfaces of them first and then the rails and spindles. What winds up happening is the flat get another coat from us doing the rails, while it has been dwelling the whole time. Then we just rinse. We do use a booster pump to rinse, so that helps greatly with deck cleaning.