paint prep

getting ready to paint and was wondering..........do i need to sand all the old paint off :?: had planned on doing that. if i scuff it up real good will the new paint stick as well :?: or am i better off priming bare metal :?: after priming is there any thing that i need to spray over the primer before painting :?: last Q. anyone know where to get a left rear 1/4 for a 66 cuda :?:
thanks for the help
sweaty