The concentrate did an OK job. My front porch and deck had several layers of paint and I found that it took up the entire jug to do a small amount of work.
Gary Recommends This Simple Green Product
Simple Green makes a good product- but since it's biodegradable and safe around pets, landscaping, and your pressure washer's seals it won't be as aggressive as acid-based deck cleaners.
You will get the best results if you wash the large crud off your deck with plain water; then lay down a layer of the deck cleaner and let it work for a bit. You shouldn't let it dry on the surface, so how long it sits depends on how hot or windy the day is.
Then you can rinse the deck cleaner off- you should see some pretty good results. You may need to do a second pass if the deck has been neglected for a long time.
It's also a good idea to give the landscape plants around/under your deck a good drink of fresh water before you start with any soap product- it will keep the roots and leaves from absorbing any of the cleaner.