I derusted an old English rook rifle years ago to engrave. It had a solid patina and a number of pits I was going to fill with soft iron inlay. I soaked it in Steel White as sold by Brownells, a solution I successfully used to derust some Model A body parts.
When the action came out of the solution, it was like extremely fine Swiss cheese, because the patina had eaten into the surface.
I finally used a small engraver's punch and chasing hammer and beat the whole surface area down a few thousandths of an inch to "seal" up the pits before polishing and engraving. Still had the larger pits to fill but they were by this time very clean. The project came out really nice even though taking a lot more time than I had planned.
