This afternoon I was installing a spring kit on my new M&P 340. While removing one of the screws for the side plate, the screwdriver blade moved a little sideways and scraped a sliver of frame metal at the edge of the screw hole. Wouldn't you know it? It is the uppermost screw, the one not covered by the grip.
Anyhow, I tried using a Birchwood Casey paint stick without good results. As a last resort, I tried a blueing stick. I assumed it probably would not work, since blueing is for steel and does not work on aluminum. But to my surprise, the blueing (actually matte black) stick worked and delivered results as if it had been applied to steel. The mistake I made is not even visible now after touching up. What a pleasent surprise!