and no I ain't kidding...
..... a byproduct of sugar refining called 'treacle' is a known rust remover for ferrous materials (iron and steel).
Treacle is also the key ingredient in a much more expensive commercial product called "Rusteco"... (that you may find at your hardware store).
It takes a week or two.. but its dirt cheap and eats rust without eating good paint.
Here's an acount of it.......
I read an internet article about using horse "feed grade" molasses mixed with water as a rust remover. I thought "yeah right", but decided to try it anyway. I mixed molasses I got at the feed store with water at about a 1 to 7 ratio. I stuck a hopelessly rusted Honda XR70 tank I got from a friend into a 5 gallon pail of this mix to see what would happen.
After about a week - no progress at all, I figured it was a bunch of crap, but since there's no work involved and the tank doesn't need to be anywhere I decided to let it sit. So today (about 2 weeks from start)I went out and checked it again.
The mix looks and smells gross! Past three days have been pretty warm here and it's starting to ferment, stink, and "skin" over - Blech! But when I pull the tank out of this "cesspool" and look inside, Lo and behold....it's a thing of beauty! The inside of the tank sparkles, only a few rust spots left, and they are soft and disappearing. All of the not-so-nasty areas of rust are completely gone and the paint only flaked loose where it was barely sticking to begin with (The original Honda red paint appears unharmed). Keep in mind this tank was considered "junk" and unsalvagable when I got it. I'm going to give it another week or two to get the really bad spots.
This method has produced the best results of the 5 I've tried on tanks and really works beautifully (as long as you aren't in a hurry)- No "etched" look either, like with Muriatic acid. The metal really shines. It cost me 5 bucks for about a gallon and a half of this molasses (I had to buy enough to make it worth the feed store owner's effort) I still have a lot left over.
I think the secret is you have to let the mix sit long enough to "get stinky" and it was too cold outside before for that to happen.
The "Rusteco Super Iron out" worked to an extent, but not as well as this, and its way more expensive
So there you go... $5 worth of molasses and 2 or 3 weeks wait would do you.