Really, this is not a problem. Steel bikes will, of course, rust if you leave them outside for weeks at a time in all weathers, or store them in a canal, or something. But with even moderate care they won't rust much. I recently acquired a 1984 Raleigh touring bike. It clearly hasn't been treated with kid gloves, there are chips and scratches everywhere, it was filthy when I got it, the drivetrain was pretty much clapped out. But there is virtually no rust whatever on the frame, the chromed forks had only tiny spots, easily removed, the only real signs of corrosion were superficial oxidisation on the threads of the bottle-cage braze-ons - no problem. This is a 28 year-old bike with a lot of miles on it. It is absolutely fine. No shower of rain is going to do it any harm.