Liquid Rubber Product For Sealing ?
 

There is a Modern Liquid rubber called GOOP sealant/adhesive in the
glue section of the hardware store

I LOVE it! It STICKS things together irrevocably. I use it to repair
evrything.

It IS goopy, but in the 12 hours to takes to harden it "flows" and
self levels.

The SAME stuff is sold as SHOE GOO and I use it to repair the "tennies"
Dries to pure rubber.

In outdoor applications a UV resistant variety is offerred as Marine
goop

Yodar