Well, for better or for worse, there is still a nice coating of rust on it since I had to set the project aside about 3 years ago. Most of the sanding work I had put into it is now covered back up. I remember the original rust being a fairly even coating, feeling like a fine grit sandpaper to the touch and sanding off easily as a powder rather than flaking. You mention that rust can - sometimes - be its own conversion coating. I would imagine that if you could put up a barrier between the steel and oxygen, the existing rust damage would already be done but you would prevent more. Does existing rust pose a threat to the underlying metal (other than what it's already attacked)? My knowledge of metallurgy is limited to a 10 week material science
course I took several years ago, so I'm not sure - can rust eat its way further into the surface even without access to oxygen?
I'm not overly concerned that there has been so much corrosion that the fuselage is structurally unsound. My concern is that in 5 years, 10 years, 20 years, it will get to that point, possibly with me or someone else suspended a mile above the Earth. I don't necessarily expect this airplane to be flying in 200 years, but I would sure like to put a lot of time on it and hopefully pass it on in one piece. The engineer in me likes the idea of the proof testing, and I think that's something I'll do with whatever fuselage I use.
I had planned to blast the fuselage just prior to coating, but the baking soda is a new idea to me. I will definitely try that. I wasn't going to use anything too abrasive - walnut shells or similar are what I had in mind. I'll add the baking soda to the list.