I've found Birchwood & Casey Super Blue fluid in 90ml bottle a very good product over many years for small jobs. You can reapply it a number of times to get a better match with a water rinse and gentle wire woolling between each. It won't be perfect but quite acceptable, and on an older gun becomes part of its character, a vast improvement over rust or bare metal. Furthermore, it can cure over time once oiled in. I last did a small patch on a 97 integral mod assembly which became almost invisible in the following months.