I must confess - I will occasionally shower a kite or 2.

However, it's only after coming home from a wet and muddy field (or that silt-like black sand that we can get on the beach here in the PNW during winter) where the kites clearly need it. I just use water, but I do it while the kites are still wet - it comes right off. If you allow the mud to dry, well, thats a whole different story I imagine.
-Tom