There is foam gasket material between the rail and the body. When I put my Barracuda back together, I first scraped off the old stuff from the rail, and then just went to Lowes and got a roll of foam peel and stick window weatherstrip. It was about 1" wide and the foam was about 1/4 inch thick. It worked great, and it was cheap. Also, if your roof rail gaskets are old and compressed to the point where they are thin, that won't help. Even with the new foam peel and stick gasket material, you might need to buy new roof rail gaskets.
