Paint will protect the outside of the tube, but a few coats of car wax (or furniture polish) won't do any harm. I use wax as a rust protector for non-moving parts like cables and bolt heads.
Framesaver is the best thing for inside quality steel frames. On my beater, I just spray WD 40 and it seems to work, but this doesnt mix with framseaver, or with some bottom bracket cartridges (the ones with replaceable bearings).