NuMetal?

I use "metal 2 metal" on places like roof to quarter seam. I don't recommend "all metal", not very good stuff. Never used Nu metal so I cant comment on it but im sure its a similar polyester resin with metal mixed in. Should be waterproof and would be fine as long as the prep was good under it.
I would sand that out in an area to see what your working with. There is a reason why painters don't like to paint over other peoples bodywork.