I had a black aluminium seatpost to strip - nitromors didn't touch it and neither did oven pride.
I don't know if it had some other outer coating, or if the aluminium was actually dyed black somehow.
I sanded the post a bit then tried ovenpride again and it seemed to work better this time, as if I'd removed a protective layer.
Still had to use wet & dry to finish off the post, I can imagine that it would be trickier with a chainring though.