what colour is the car now? if its black rubbing down the paint now will be ok to go over it but if you are changing the colour of the car I would suggest priming the whole thing first. Just make sure you know what colour you want and go with that it, its your car so its your choice. Im a person that needs some kind of effect in the paint, it maybe metallic or pearl cuz plain colours dont do it for me sorry. Just take your time in deciding dont rush things it will still be there tomorrow