You have to be kidding. Leave it long enough and it will clean right through to the other side. Yes it fetches off flakes very quickly but then it peens the metal too. I also use a Turk's Head knotted brush (cup style rather than flat) and very good they are too, but they can sometime just polish over things and don't always rip off lumps. The needle gun is really good for awkward places.
Personally I would not use a needle gun on the flange and backing plates that you are showing there. I'd use a brush too. But for heavy rust, the NG is the medicine that's for sure.