I did this with my gas tank, but no reason it wouldn't work elsewhere.
I started de-rusting my tank the "vinegar route", but still had rust inside even after a month of sitting. Needless to say, the tank was pretty rough inside. I went down to the hardware store and picked up Naval Jelly, Locktite makes it. I poured a 16 oz. bottle inside and shook the tank around to let it coat the inside. This stuff is pretty viscous, and will stick wherever it lands, so you don't have to put a whole lot of effort into getting the inside coated. After an hour my tank looked great on the inside. Try for yourself and see.
I would suggest doing the vinegar solution first, and doing this to account for the inevitable flash rusting that will occur afterwards. The Naval Jelly will leave a coating that will not rust.