Painting a live steam loco

Well I have a ruby that I have previously made into a forney, but I have decided to make it superdetailed, and in the process repaint it. Well I know I have to clean it of all the steam oil with vinegar or such; but do I have to strip the locomotive of the black paint that's already on it and bring it to the bare brass and copper (I'm nervous about that because I want to repaint part of the boiler as well), or rather could I rough up the paint a bit with steel wool so that the new coat of paint bonds well? Thanks.