I used to visit my brother and family, and get up early on Sunday and go just a short distance away to meet the local cycling group for their big ride. I had my bike downstairs, where it was pretty cold from the AC. When I'd take my bike outside into the warm humid air, the whole bike would almost immediately get a very visible coating of condensation, which I would assume would also get into the nooks & crannies, and possibly even inside the tubes.
Also, I recently re-used a rigid steel fork from a relatively new and low-miles bike, on another project, and I noticed a fair amount of rust inside the steerer tube, and there was a hole from there that went into the both fork arms, themselves. So, I'm pretty confident there'd be some rust in those, as well.
I'd say internal rust is a very legitimate concern for steel frames.as
As for a protective spray product (other than some specific frame saver-type product), I think I'd use the (now discontinued?) Dupont Multi-Purpose Teflon spray that I still have a small stash of . It seems to dry in place, and last pretty well.
Last edited by Brocephus; 07-27-19 at 09:11 AM.