This was a first attempt, and I was originally expecting to replace the cylinder, so I am not that disappointed, but think that a more experienced hand might have saved it, so I am trying to learn from it.
The procedure was more tedious than I expected, and I think that 'the next time' I need to be more conservative and sparing with where I place the acid, rather than trying to remove all traces of aluminum and assuming that the plating will protect the rest of the cylinder. That is why I asked the "When do you stop?" question.
I also found it hard to see into the cylinder, especially that area above the exhaust port.
I started with 600 grit wet-dry paper which was very slow. 220 was too rough, and quickly scratched the plating. Even some of my ScotchBrite pads scratched the plating. I think that the 320 to 400 grit wet-dry paper is probably the best compromise to leave you with a lightly honed surface.