For really heavy rust, it is more economical to physically remove some of the rust with wire brush or wire wheel first so as not to deplete your rust remover or waste electricity if using electrolysis.
I've used Evaporust and electrolysis with good results. Another effective product that will not eat the base metal is Rust Beeter which is made from dried sugar beets. It is mixed with water and the rusted parts are left in the slurry for as many days as necessary. You can also buy molasses from a feed store and dilute it with water at a 6:1 or 8:1 ratio for a slow but cheap and effective rust remover. This does work better in warmer weather. It can take a week or two, but it's a lot cheaper than the $20 a gallon Evaporust.
