Only just stumbled across your channel, love your attention to detail - I have been a mini enthusiast since the 1960's when they were at the height of their popularity here in the UK, at the time I owned a 1963 model which I re-shelled in the early 1970's complete with a 1275 engine, the underside was painted with red lead paint (now outlawed being a health hazard) all cavities were injected with Dinitrol rust preventative fluid. All these measures must have been of value as I still own this mini and it is in remarkable condition considering its age having never been restored or welded, the only problem is that the paintwork has become shabby and in need of stripping and repainting. I will keep an eye on your progress, best wishes.
Thank you for reaching out. Sounds like you hit on the right rust prevention combination. That’s amazing you still have that car. I’m getting close to getting this one back on the road very shortly.