I'd highly reccomend a pressure washer. Mine was covered in mildew, paint stains, etc, and I just sprayed it down with ZEP citrus degreaser, pressure washed it, and doused it with Thompsons. Came out pretty good, probably would have looked better if I stained it, but hey, its my dad's deck, and I'm not working my hands to the bone to improve his property.
