Chain mail is not weatherproof, actually. And maybe a lineothorax would in fact be much more weatherproof (than chain mail): as it is made from dried organic material all you really need is a good coating (dye) and it should last quite a while. Coating can be trivially updated (simply scrub & repaint), on the other hand rust in chain mail tends to means that if one ring is bad so are few others.

Now the real test of resilience is of course the Indian monsoon, and we know the lineothorax didn't survive such weather. But also consider the example of much more advanced metal working of Medieval ages and look at what happened to the crusaders trapped on a river island when the Nile started flooding.