Is it caked on mud? Or is it just heavy dirt buildup like after a storm kind of build up? Unless its caked on mud, like actual dried mud pieces you can still use onr.
Mix up some onr @ a strength between QD and actual wash strength. Pre spray the panel down and some of the surrounding area. Take your onr @ wash strength, load up a sponge, mf or whatever you use. Now begin the wash. Take it slow. Make multiple passes. It will take longer than usual. Dont expect to take all the dirt off all in one swipe. Once the water is too dirty, dump it out and fill up another bucket.