A few years ago when I worked at Gander Mountain, a man brought in a Mossberg 500 that had been in a fire at his house. It wasn't burned-up, but the fire retardant that was sprayed on the home was very corrosive. All the steel parts were covered in a thick coat of rust. The aluminum receiver was filthy, but not rusty.
The staff gunsmith took it into his shop and scrubbed it in a chemical bath. The rust came off the barrel but the surface was badly damaged. When the gunsmith was done, the anodized aluminum receiver looked brand-new.