"Caulking" for Taillight

["John Corbishley" <gtr1@traverse.com>]

Having just received my replacement from the good Mr. Semple I discovered
that there is some sort of "mastic" which seals the taillight from the
outside elements.  Being a Sunday I had to re-use that which was on the old
taillight, but am concerned that I did not have quite enough after cleaning
out all the road debris.

I seem to remember this issue being addressed by the List some time back.
Is this a 3M product, and does anyone know what it is called?  Otherwise any
alternative suggestions.

TIA

John Corbs
Traverse City, MI
'83 & '85 Ur-qs (for my sins)