Some chickens just don't keep themselves clean, which causes a build-up of the white urates that you are talking about. I find that this dirtiness is more common in older chickens in cold or wet weather. Usually, at least in my chickens, it stays around for 2-3 months, and then suddenly, you notice that they are clean.
There is a condition known as Vent Gleet which causes a white discharge associated with whitish sores near the vent. Gleet is a fungal infection, which smells bad and can be very harmful to chickens if not remedied. I'd check your birds for any sores; if they have none, then your problem is just dirtiness, not Vent Gleet.
If you want to get rid of the white pasty build-up, give your birds a bath. Make sure their diet is good, and watch them for any other signs of disease, and you should be fine.