Here in Europe folks use a twenty percent solution of muriatic acid that is sold inexpensively in grocery stores as a mineral de-scaler. It also works well to remove rust from stainless steel and rust stains from fiberglass decks. I use it to get at the rust that lives in crevices around deck fittings that I can not rub or polish out.
The problem with it, though, is that in its liquid form it flows away from the rusty site and onto things that you might not want it on. You could buy pickling paste that is used by professional welders, but I found an easier solution that you can make on the boat.
Get a glass or plastic container (metal will be eaten by the acid) and pour in about an ounce of acid. Then add a tablespoon of flour and stir well. It is best to wear safety glasses and rubber gloves for this. The consistency should be thick enough so as not to flow on a vertical surface; add more flour if needed. You can add a little food coloring so as to keep track of where it has been applied.
Try to keep the paste localized to the rusty area and leave it on for about a minute or two, then rinse it off with plenty of fresh water. Reapply it if necessary. The paste may dry and harden in the sun but rubbing it off will help polish the spot. If the shiny metal gets etched during the cleaning, simply polish it with a buffer and some polishing compound.
Remember to clearly mark the jar if it will be stored, and do not use it as a wart remover.
