J.P. Weigle's Frame Saver is the classic rust protection for steel bicycle frames. Cyclocross bikes ridden in the wet/mud and that are pressure washed, all-weather touring and commuting bikes, and children's bikes that may be left out in the rain are all excellent candidates for Frame Saver.
Application can be done at any time, but ideally when you are building up your new steel frame and fork. A quick burst into each frame opening (seatpost, bottom bracket, water bottle bosses, etc.) prevent damaging corrosion.
Please note this aerosol item can only be shipped via Ground methods (no Air shipping).