If that hasn't happened to you at least once then you haven't been painting long enough. This happened to me a good few times and mostly i can paint over it straight away. But then you get times when there is to much paint on it already to put more on it so what can you do?
Usually it's a small part like a gun, sword or a head that's need to be redone and there is an easy way to remove the paint without having to striping the whole model.
Hope this helps