Here's a product that gets rave reviews from members on another forum that I subscribe to:
Evapo-Rust Rust Remover Home
For large areas and items that cannot be immersed they have had good luck saturating paper towels and applying to the rusty surface. Unfortunately I cannot get it where I live but pass this on as they think it's the best thing since sliced bread. Try your local auto parts store.
Otherwise you can use the old vinegar and lemon juice treatment, same deal, paper towels soaked in the stuff. But wear plastic gloves as it makes your hands stink. And takes a while. And for large items that can be submerged I have had great luck with washing soda/water solution and a battery charger. Just derusted a five foot add on to my mooring chain (1 1/4 inch stud link) using that method.