I got some Metal Glo (United Cutlery brand) and tried it out on an old knife really quick.
Took off the heavy, dark patina really fast. There was still this underlying patina that looked subtly like water stains or something similar. Maybe I just gotta put more elbow grease in it, we will see.
Though I am no metallurgist I think sometimes you'll get spots where the reactions forming 'patina' are much deeper and have put down roots, and maybe are not so easy to remove if at all, unless some grinding occurs. I've long since polished away the colour on my practice knife, a santoku, but there are still hints at the swirls from my erstwhile forced mustard patina. I've even scrubbed the blade with some quite abrasive stuff and it refuses to go.
It probably depends heavily on how the blade is finished. A mirror polished would probably have less of such patina than one finished at 300 grit, I'm guessing?
The metal glo works great though.
I use japanese natural fingerstones. Depending on the patina sometimes i use a combination of different stones. Drnaka has a nice post about that where he tests different methods. I have never tried with any powders because i get amazing results with natural fingerstones.