This. Buy some ONR or DP Wash & Gloss from Autogeek. Buy some buckets from Home Depot (fill the buckets with warm water) and a grout sponge and one of these to soak the panels with heavier dirt on them.
http://www.amazon.com/Solo-418-1-Lit...garden+sprayer
http://www.amazon.com/Chapin-20000-1...garden+sprayer