It feels like some sort of plastic polymer and I'm trying to access it through the interior. I removed the rear interior but there's a large metal sheet that's keeping me from accessing the exterior panel from the inside. Any help is appreciated, thanks.

Exterior panel that appears to be a plastic polymer
Metal sheet blocking access to the exterior panel

 

You don't have to acces it from the internal. if its plastic you can repair it with the same plastic material and a heat gun, but you must find help in order to determine what kind of plastic is this.

after welding you can apply fiberglass enchanced body filler and then the ordinary procedure. You can add fiberglass body filler without plastic welding but there is the danger of damaging the paint after a lot of miles driven.