The Alien face is right; I use to live in Santa Barbara California and one of my bikes-the beater commuter-was a low grade steel bike that sat outside for 10 years in the rain because I lived in an apartment and had my good racing bike inside and there was only room for 1. I also rode that commuter bike on the beach where the tide was at and it would get drenched with salt ocean spray. I would take it home and rinse it with fresh water but I never treated it with any internal stuff, only waxed the outside about 4 times a year. I still have this bike, though in total disrepair. There is some minor surface rust on the rear and front dropouts and where the paint is scratched on the frame. but nothing even close to serious. This bike is now 22 years old.

My other steel bike is my main bike and it to is now 19 years old and it has no rust on it anywhere and it was never treated with any internal treatments either.

As Paramount said, keep it waxed and touch up scratches. If your going to sleep over the rust issue than treat the inside of the frame with Frame Saver or BioShield; but the frame will outlive you.

WD40 applied to the outside of the frame is not a good idea, since it attracts grime and dirt like a magnet than you have to wash the crud off. Just wax it with a good (Mothers or McQuires) auto wax.