in areas of the world woith lots of salt on the road (for example Germany...) we use this wax or grease like stuff in frame rails. Not sure if available in the States, but I had GREAT experience with "Mike Sander's Korrosionsschutzfett". Essentially it is a grease which you heat up until it gets liquid (almost like water), then spray it in with a 360 degree nozzle. When the sun burns on the car it starts to melt again and digs into any rust which has redeveloped.
essentially you deepfry your car...

The best enclosed cavities rust the first. You need to get the water out of frame rails! It will always find a way in (air moisture if nothing else...), if there is no way out and no wax / grease to poush it away you get rust.
damhik...
Flo


1967 Fury III 4door HT tiny 318, late model roller block, bored and stroked to 344