Guns often respond to restoration attempts. Don't give up, keep trying. Rust is weak and falls away over time, just like it grew.
Our family rust story involved an 1860 Colt Army. It had been promised to my father back in the 1930s. It was owned by one of his cousins and was in nearly perfect condition. But after WWII, the cousin died and my dad tried to claim the gun. As often happens in families, the gun couldn't be found. Usually that means someone of higher rank just took it. So my dad did the discrete and honorable thing, he let it drop. Then about 1960 he got a phone call. They found the gun under the front porch. Terrible rust (much worse than the gun shown here.) Of course my dad wanted it, so we made a trip to recover it.
The story was the young teen kids of the owner (or his grand kids, its unclear) had played with it and just left it outside. For years. So we took the gun home. And dad and I started our project, just like so many others over the years. My brother had no interest in guns or rust, so he left us to our gun. I'd been reading in Popular Science about a then great product, Liquid Wrench. The local clip-joint/ discount house carried it. So off we went to buy some. We discovered it was sold in tiny oil can like size, and then in big bulk cans. Dad opted for the big size, since the price was pretty similar.
We submerged and soaked the entire gun. One grip was gone to the elements. Soaking does wonders, but it takes time. Its one thing we had plenty of back then. Besides, we had plenty of research to do trying to find springs and small parts. Within a few weeks we managed to get some of the screws out. Dad was afraid to use heat. He finally gave in and we began the more dangerous part of our project, heating the soup.
We weren't after a lot of heat, we just went out in the yard, plugged in a hot plate, and began bringing up the temp. While we were afraid of a fire, there wasn't enough to damage any metal. Repeated hot cool cycles managed to work the penetrating oil down in side and we got the center pin and cylinder out, then the barrel wedge. Lock parts were in better shape than we'd hoped.
In less than a full year, the gun was up and clicking. We didn't have the nerve to fire it. Old steel and heavy rust were just too scary. But we did make the missing grip out of a piece of walnut. In the end it wasn't as pretty as it once had been, but it was nice. After my father died in 1980, my brother wanted it (in contrast to his refusal to even spend one evening with us trying to restore it.) Things like that happen within families. Now looking back, the real value was working with my dad on the project. Once the task was finished, it was almost forgotten except for the time we spent together.
Its how you make something out of almost nothing.
To me the gun this thread is about shows great promise. This isn't a mission impossible. It'll take more time than the gun is worth, but its fun.