Simple Green and a pressure washer has worked well for me on plastic chassis parts like the shroud. On somewhat nasty under car parts like rail mounted splash shields I have used a light coat of Krylon semi flat black which seems to hold up well. I wouldn't do that on a shroud as it might just flake and look crummy... if needed I'd use the 3 step SEM plastic paint process (cleaner, adhesive promoting primer wash, black finish coat).