When I rust blue or brown I remove all removable parts and/or pieces. Reason being, it's very important to degrease every nook and cranny on the piece being blued. I finish the small parts separately. As far as the barrel flats go, they can be finished to match the original (whatever that is). If you don't want to refinish them, don't apply the solution to them. Or they can be masked off with clear lacquer.