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Parkerizing is a finish applied through an acid bath. The military uses it on its guns (can't be applied to aluminum so not AR's, think WWII guns).Cerakote is a baked on ceramic coating. It's been proven to be very tough.Bluing is a generic term for several processes but basically they all leave a thin coat of oxide on the steel. It's not very rust resistant compared to the other two because it is a form of rust.
I do not thing water can get under cerakote. Check out gorge custom coatings, they are a sight sponsor and did a great job on my gun.
I got my old over under cerakoted this year and it looks awesome. I went with midnight blue and gorge custom coatings did a great job, ended up being about 100 bucks including shipping both ways