What is the putty like coating used on the Audi rocker panels?

[syljay]

I'm de-rusting a junkyard fender which will replace my damaged front 
fender.

On the lower surface of the fender and on the lower inside, Audi used 
some kind of putty sealant to protect the metal. The stuff is 
semi-hard and is still flexible.

This stuff reminds me of the Phenoseal stuff I used to cover cracks, 
etc before painting wood or walls in the house.

I want to get this stuff to protect the lower fender and the inside 
bottom of the fender.

I think body shops use something similar to seal seams and fill gaps 
in places that are not cosmetically important.
And, this stuff can be painted over.

Any body know what this stuff is? Got a trade name for the stuff?


SJ in NJ