What is black coating on the sheet metal behind the license plate (underneath truck b

The controller box for a third-party trailer wiring connector we got (4cyl; no factory tow package was available) is attached wherever you can find a spot with a double-sided foam adhesive pad. The truck is getting a bit dirty, so I figured I'd better wipe down the flat spot I found right behind the license plate to make sure the box stuck securely.

I was a little surprised that my first swipe with a paper towel moistened with denatured alcohol seemed to be very dirty and black. The second and third even more so. Oops. It looks like I managed to wipe off some sort of coating and got down to bare sheet metal. (At least the controller is securely affixed!)

Should I spray paint the now-bare patch with enamel (e.g., Rustoleum or the like), or is there something fancier I need to get to avoid rust? I wouldn't have expected normal paint to give up to just alcohol so easily.

I searched the forum for "rustproofing" and it seems there are some oil-based products I've never heard of... perhaps this is one of them?

My preference would be to come as close to replacing what was there as I can. The spot I cleaned was about 6"x6"; the box is covering about 3"x4" of that.

Thanks!