Ok if you are going to be applying bondo or Plastic filler you should apply it to bare metal that has been ground a little so that it will "stick" or "grab" onto the metal. Sometimes it is a waste of primer applying the bondo or plastic filler after you primer due to having to rough it up so it will stick. As for the new 1/4s you dont need to take them down to bare metal. Just need to rough them up so that the primer will stick to them. Just have to make sure that there is no surface rust starting when you go to primer them. Hope this helps