A reliable method for removing light surface rust from blued barrels, used by English gun trade is to coat the barrels with a light oil, then using chalk sticks [as used on a black board]
scrub the barrels with the chalk sticks, the rust is easily removed without damaging the blued finish.
This method can also be used to brighten the finish on Damascus barrels that have over time developed a dull surface finish.

Roy Hebbes