Phosphoric (I think that is how it is spelled AKA "Ospho") Acid is a rust killer that converts rust in to a primer. I usually buy mine from a paint shop such as Sherman Williams or maybe home depot. This stuff works great I just spray it on with a bug sprayer and let set for a while, then rinse with water. It is also environmentally safe and can be found in
coke-a-cola. It will also clean your metal prior to painting because it eats dirt and calcium. It will turn all rust black and is what is used to create rot iron. About $20.00 a gallon and can be cut with water up to 50/50 and still do a great job.
In my opinion, it is the best way to go especially on a large project.