I did not use primer, I don't think it's necessary on the external surface. I thought it would be best used on the body filler. So I did a peel test and when removing vinyl that had been applied over body filler and set for over a day the wrap was very hard to remove and you could even see that it has lifted a very thin layer of the filler material. What I am most concerned about is the adhesion to the back surface of the panel. I did a lot less work on the back side, lightly sanded to 80 grit and spray painted with undercoating. I don't think my wrap is going to stick very long to this surface so I'm planning on adding plastic edge strips and/or the primer to the back side. If someone has a better idea I'd like to know about it.