It’s called panel adhesive, any parts store that has a decent paint and body inventory should have it or at least be able to get it. The only downside is usually it comes in packaging that automatically mixes the two parts as it gets dispensed and usually the maker has a special gun with two plungers similar to a caulking gun and they’re proud of it (cost). I’m sure for a one time use such as veekay’s use he could get creative and come up with a way to use it without the special applicator gun.
The manufacturer used it on the cars that have plastic or fiber panels glued to medal attaching pieces.
The back side of the fender needs to be free of rust before use or he just Pi$$ing in the wind. And I’m sure the next owner will cuss the Prior owner for their backyard bodge.
I read some of y’alls post on here and think of when I built my log home from scratch using modern tools instead of draw knives, adze, hand planes or any other traditional tools that the settlers would have used and some of the smarta$$ comments from people and my response would always be do you think for one minute if chainsaws or electric sanders or any other modern tool had been available back when that they would have used the labor intinsive tools that that’s what they would used,