I have had my Desert Eagle since 1991. (Made in Israel, IMI) It's the Mark VII version... and, not the Mark XIX you're inquiring about. Bought it from an individual that had only put two magazines through it and complained about its weight. It has a black slide / barrel and aluminum lower.
In the many early years, (a little over a decade), carried it on my hip in a cheap holster made for the gun while hog-hunting on a deer-lease. Exposed it to rain, mud, and whatever else the trail threw at me. Cleaned and oiled it thoroughly each evening once I arrived back at camp. Have never had a problem with rust from the gun in many years of carrying it in some pretty bad environments.
There is very little fading of the paint on the lower left-side edge of the barrel toward the end where the corner sat against the cheap holster and my leg. It was carried and kept in the cheap holster for many years. (Family-member constructed a very nice drop-leg platform within the last few years in a laminated-denier that resembles leather, and I've not had any further issues.) I have a friend who has recently began to Cerakote and Hydro-Print for a living and have been thinking about giving the gun a new coat of paint. Either that, or just Cerakote the slide and barrel back to black... aluminum doesn't show much age. It doesn't need it, but I am pretty particular when it comes to... well, anything.
That's my experience with a Desert Eagle, made in Israel, and in the Mark VII flavor. Hope it offers some assistance in reference to your question.