Quarter Panel Adhesive
 

So I replaced my rear quarter panel a couple of weeks back. It was surprisingly easier than I thought it would be, save for one item. The adhesive used to attach it at the top to the roof line. There are two different types of adhesive: one that attaches to the inner skin- around the window and the like, which stays tacky. That one was no problem. But the other one that attaches to the roof line is one that hardens to a pretty stiff consistency.

Anybody have any idea what kind of adhesive is used for that part of the assembly process? Any idea where I could get my hands on some?
Am I over thinking this and should I just go with some RTV instead?

Any knowledgeable recommendations appreciated!