From my SAR days many years ago, I remember that the aircraft were given a fresh water wash at the end of the day if they had been flying over the sea and a foam wash on a scheduled basis (once a fortnight?).
After the foam wash the aircraft were treated with PX24 (same as WD40). That's why the soot sticks to the fuselage.
I can't imagine anything changing with that regime. Professional pride has kept them flying in relatively good condition considering the environment they operate in.