Again, here is my 2 cents on rust. I do travel down to Costa Rica a few times each year, as I have a small parcel of property on the Pacific Coast. I always take my AD (double cut) and either a SH-E or BM-E(both black coated). During the dry season, rust is never really an issue, BUT during the rainy season it's a different story. MY AD has never really had any real, hardcore rust spots on it, but if not used or inspected every few days, it will show very small rust pits trying to form. These are easily taken off with a bit of light sandpaper, but if I didn't do anything about it, I would imagine it would only get worse. Daily use with it helps keep the rust spots off along with the sanding. On the other hand, my Sh or BM with the black coating does not as easily form rust spots, as I use them daily down there also. Using my blades is the best insurance on keep rust a bay in the tropics, as I always try to keep them dry after use and inspect them every few days. Now, all that being said, my brother also has a BM-E which has stayed down there for a few months boxed up, but rust can and does form underneath the black coating, which makes the coating crumble off in chunks and can leave a good coating of rust, which we remove with a wire brush wheel on a drill. It's not that big a deal to him as it really doesn't alter the performance, but now a days, he brings it back so as not to worsen it more. Also, I have a friend here is LA(Lower Alabama) that goes deep sea fishing with me alot, and he has a NO-E(black coating) that he cleaned some salt water fish with. He's not really the best at keeping anything in good condition, so now his NO-E has huge chunks of black coating crumbled off and THICK rust all over his NO from putting it up WET. Again the wire brush wheel was the only option to remove the rust, and performance is not affected, but looks are. So, to wrap this essay up haha, a coating of any color will help keep rust off, but if neglected even the coatings(black from experience) can flake off in a hostile salty environments. I think even Jerry said that salty humid environments can rust plastic, and he is exactly right. So take this as only my two cents with a bit of experience with two coated ergos and one double cut AD and months spent in the tropics and on the salty Gulf of Mexico
