I have an old nickel plated S&W .357, I bought at a police auction 35 years ago. Looks good, even after having a lot of hard use. Then it came up missing for a year. The wife had forgot and left it in a fender bag on the 4-wheeler. It spent one year in that non waterproof bag, with the 4-wheeler sitting outside the whole year. Rain, snow, and shine, cold to -55, hot to 90 deg. When I finally found it, it was sitting in water, it had rained that day. I pulled it out and yellow water ran out of the frame. I opened the cylinder and took the ammo out, the brass was discolored and the bullets were corroded with a white coating.
I disassembled the gun. All the interior parts were rusted and unmovable. I took them to the wire brush on my grinder and cleaned them up. I took my dremel tool to the inside non plated surface, and cleaned it up. I used an oily rag and wiped the outside down. The rib on top of the frame and barrel has a grooved pattern, and to get the rust out of that I used a brass wire brush. The inside of the barrel and cylinder were covered with a yellow coating, but after cleaning, both were without pits and looked like they did prior to getting wet.
After cleaning it up, and a good oiling, I gave it back to the wife. It looked just like it did when she commandeered it 30 years ago. She never knew where I found it, or the condition it was in, but she is happy. That is her favorite pistol.