Have you tried cleaning them up first? Once they get claggy, they tend to stay that way or get worse IME, even if their poop has gone back to normal.
If you soak their bums in a bucket of warm water, you should be able to loosen most of it and clean things up. Maybe also trim the feathers around the vent a bit if they have very fluffy bums. You may well find that if you can get them clean, they'll then stay that way of their own accord.