factory undercoating?

[jblueridge]

I getting ready to cut my inner fender panels to remove rust. There is a layer of black, tar like undercoating on top of a thick layer of white or cream colored stuff: polyester?

What did the factory use?

 

[jblueridge]

Here's a photo of the cream colored stuff. You can see it where is got the top layer of tar off.

 

[divemedic]

I'm cutting rust as well in various spots. I was certain that it was bondo or some other body filler.

 

[jblueridge]

It is fairly evenly spread out. Seems rather soft.
I will re-coat with brushed on 2-part epoxy primer, then some top coat, then asphalt roof cement.