My 68 Cutlass had new rear quarters put on. now the seam between the lower front of the quarter panel and the rocker panel needs filled in, I just don't know with what. Whats the procedure? How did the factory do it? On my other car (which is original paint) it is just a line not really a gap like on this one.
Just to clarify, this is the area right behind the door and it follows the line of the bottom of the door.

 

with the area in primer put tape on both sides of the seam and use a good quality 2 part sandable seam sealer. spread it into the seam pushing with your finger for pressure to get it right in there...then wait a couple of minutes and pull the tape off...a sandable sealer gives you room for error...then paint