All were factory corrosion proofed during the RA building process as a standard
Well... yes.... but.... The Rheims build Cessnas I have seen do have factory applied zinc chromate on the internal surfaces. But, when I saw one which had been partly disassembled for repair, I noticed that the area between a fuselage bulkhead and the skin was bare, making me believe that the factory zinc chromate was applied by spraying after the fuselage was assembled. This really compromises the effectiveness of the corrosion proofing. The legacy Wichita built Cessnas were sadly bare inside, unless they were ordered with a factory float kit, in which case they were properly zinc chromate sprayed. The zinc chromate application adds weight and cost, but it's worth it if you want the plane to last.
I bought a damaged flying boat in England about ten years ago. When I got it into my shop for the repairs, I had to drill out some rivets from the keel behind the step. As I drilled, suddenly, I hit the dreaded white power in the drill chips. I finished removing the damaged frame, and found that both it, and the inside of the hull skin to which it had been riveted (both of which were factory zinc chromate primed) were perfectly corrosion free. During a prior repair, someone had inserted an unprotected 1/16" aluminum shim, and it was corroded into white dust. I replaced the damaged part, with an original new part (which was zinc chormate primed, and the repair was fine. The airplane was otherwise free of corrosion, aside from one damaged area where the bilge water sat.
In North America, when considering the purchase of a legacy GA airplane, where on the continent the airplane has spent much of its life is an important consideration. Coastal/Great Lakes area airplane, really thorough corrosion inspection, and prepare to walk away. Airplanes from the inland areas are generally much better for freedom of corrosion.
A local airport in Toronto (which sadly closes forever next week) had a major highway build along its western edge 30 some years ago. Since then, the airplanes which are tied down outside there were really corrosion vulnerable. In the winter, salt is applied to the highway to melt snow and ice, and the resulting mist of very salty air is carried by the prevailing west wind over the airplanes at the airport. I have seen some horribly corroded airplanes which were neglected there.