Sound Deadner

[LDO]

So last night, I had a little extra time so I figured I's start taking my doors apart and order some weatherstripping, etc. After pulling the upper door panel, it looks like the previous owner glued sheets of 1/2 inch styrofoam to the inside of the doors!
not sure what that's all about but it took me a few hours of carving the glue off with a razor blade....

No rust currently exists inside the door and I found some rubberized undercoating/silencer at PepBoys. So, my plan was to spray some extend rust inhibitor stuff inside, let it dry then a light coat of this sound deadner and put the doors back together.

Anything wrong with my plan? I've heard mixed reviews on using undercoat and stuff so looking for advice/opinions.
Thanks!