If you want to do the job yourself there are a couple of ways to go about it.
The Rust-Mort that was mentioned earlier is a good way to go. You will need to wire brush off all scale rust and then apply. You can either brush it on, or in larger applications you can put it in a spritz bottle and spray it on. Once you apply it you will need to let is sit for 24 hours. It takes that long for the phosphoric acid to do its job. It will usually form a powder that you will need to wipe off with a damp cloth before painting. Once the water dries you can topcoat. Hammerite or a zinc rich (cold galvanizing) paint like AuntyM suggested would both be good ways to go.
Be warned that Rust-Mort has to be topcoated, if not it will rust again. If you dont want to topcoat then you will want to use Rust-Seal. It is made from the SEM company just like Rust-Mort but has a different chemical in it that forms a protective coating. Rust-Mort runs about $13 a pint, and Rust-Seal about $23 a pint.
The other methods require complete rust removal and spray equipment. Email me if you want to go over those.