east wood makes a awesome product for the inside of the frame called "internal frame coating " . it comes with a 2 foot hose and nozzle to coat the inside of the frame , there is a video on eastwoods site of it .
but for the outside of the frame there is no better product then POR-15 , wire brush all the loose rust off , prep it with the "marine clean and metal prep" and paint on the por15 and hardens like steel .
i sent my rear diff out to get sandblaster and then painted it and the frame and every thing else with por 15 and it looks amazing , like a brand new truck .