When I was working on research ships which spent a lot of time at sea, we found that traditional preparation of metal surfaces wasn't working. Some of our deck equipment would rust up in a month, after being taken down to bare metal and primed, undercoated, then two finish coats applied. We found that taking down to white metal, then applying Etchbond or a similar product (acid to etch, some sort of thin rubbery stuff to bond), one coat of Zinc Chromate, a coat or two of red oxide primer, then undercoat, and two finish coats, was definitely the way to go. One particular winch drum, on a very wet deck, would rust in less than a month, and have big rust flakes formed in three months. After treating as described above, it went over four years without needing to be stripped and repainted. It may have been longer, but the ship was sent to the East Coast, and I lost track of it. I have used this system on truck winch bumpers and other rough use steel items, and it lasts far longer than any other paint I have used. If you want your door to look good for a long time, use this system on it. Etchbond or similar products, zinc chromate, and red oxide primer, can be found in marine paint store.