I was reading in one of my Field & Stream mags and they mentioned that using a #2 lead pencil will work.....Take the pencil and rub it over the rust spots covering as much as possible....After the rust has been covered with the graphite from the pencil remove it with the eraser and the rust will come right off...
This also will scratch the barrel but the rust will be gone. Then a re-blue is in order.
Also some rust is harder to stop than others. Sometimes it will return no matter what you do. I had one gun that even returned after it was completely re-blued. Took a year but it came back in the same place.
Get a product called Frontier 45. It looks kind of like steel wool. Should be able to find it at most gun stores. Put some oil on the rusted area and rub the frontier 45 over the damaged area. Wont hurt the blue.