Need help fixing seam on the firewall of my 67 camaro

I have a 67 camaro, and on the firewall there's a seam that runs the full length of the firewall. It use to have some kind of caulking,
tar, or something that was in it that was plainly visible and ran the full lenght of the seam. I grinded it out to fix the firewall, but now I want to put it back before I paint it. Does anyone know what it was so I could buy some and replace it?
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