Use fine steel wool saturated with oil, I use REM oil. If it is rust only it will come right off. But as stated above it looks like rust where the finish has come off. Clean it up and if it is bare metal re blue. I have had good luck with birchwood Casey perma blue. Follow the directions and repeat until the finish is where you want it. Then saturate in in oil and let cure the for a few days, applying more oil every day.