Need help with some janky...

[68Blu]

I'm cleaning up the doors on my 68. After removing the panel I found some hard black "foam" has been sprayed on. I would like to add some sound proofing but there's no way it will stick to this stuff due to its roughness. Any ideas on what it is or how to remove it?

In my pre-post search I noticed some of you mention adding a sheet of plastic as water proofing before replacing the panel. Any special grade or just cheap painters stuff?

Thanks