<VV> black tar-like stuff inside quarter panel

[John McMahon]

I recently removed the RR quarter panel from my 1965 coupe.  What exactly is
that black tar-like substance that is at the bottom of the panel where it
attaches to the rocker panel?  Tar?  Any idea what to use for a substitute?
Tar?

Also, there is a kind of sealing material stapled to the support that is
behind that middle of the panel.  How necessary is this?  I destroyed a
chunk of it getting the panel off.

Thanks,

John McMahon
1965 Monza coupe