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Even the BB doesnt look bad at all, certainly not at respray level IMO,so that would suggest to me has been quite well cared for before you. I agree with Lolo, might be worth just having a look inside the BB when its next out and running your finger inside the tubes. I also think a drain hole cant do any harm. Plenty with better knowledge than me on here though.
My 1979 Holdsworth (owned from new - srs paper round money that) had very little rust inside when I had it resprayed by Argos in 2014 and that had been left out in all weathers during its life.
velosaurus-
When we replaced the BB we were also retapping the bottle cage mounts and a lot of WD40 went through the downtube. All came out very orange and rusty... Certainly nothing feels to the extent of major/flaky rust though. It has been inside for the whole time I've owned it, I imagine that atmospheric moisture is enough to cause a thin coating of rust on the inner surface over 40 years (or thereabouts).
Catriona
I haven't treated the inside... Is there any use in using frame saver on an old frame? I got the impression that it was something you would only do when new, but would like to be proved wrong there. The BB shell cleaned up very nicely after some WD40 and brass brushing with minimal rust, but obviously impossible to know about the rest of the frame.