Digging up an old thread as I've been busy doing the same, cleaning out the wheelarches and arch lips. The rear arches were caked in mud, especially on the trailing edge, the leading edge not so much.
If you haven't done this, I'd recommend getting your fingers into the wheelarch lips and clearing out the muck - plus, both the leading and trailing edges of the rear wheelarches are ready-made muck traps, just waiting to cause you grief.
I tackled the rear initially with just a hose and water, then sprayed with a dilution of Bilt hamber Surfex HD and scrubbed with a bug remover sponge, although any all purpose cleaner would do. Will let the area dry overnight, and will treat tomorrow with liberal coatings of Bilt Hamber Dynax UC (underbody clear) wax.
Are there any 'gotchas' removing the front wheel arch lip protectors?