Panels that are 90 degrees to the ground, like the one shown here, gets much less dirty over time compared to those at a lower 45 degree angle. To keep these signs looking their best, only clean them when they start to get dirty from bird poop or tree sap. This does not count with graffiti. What we do is get a small ice chest with Dawn dishwashing detergent and water. Save your 20 oz soda bottles for the perfect rinse bottle. Rinse and repeat as necessary.
We have dozens scattered across town and some only get cleaned a couple times a season while others under trees will get cleaned much more often. Only use Dawn as it is a very mild detergent and seems to do the best job. We use a regular wash cloth, as a SOS pad or other similar products will scratch the surface of the panel.
For more info on panel cleaning or to volunteer on our Battlefield Crew, email firstname.lastname@example.org.