I'm Heartbroken. Time to face reality that my Cuda is a parts car.

Flux core IS trash. And yes, for a guy that has to spend a lot of time replacing panels on his car, that’s completely relevant.

You’re the first person I’ve talked to in almost 30 years of working on cars that wanted to use flux core wire for anything other than a paperweight. Seriously. I don’t think I’ve talked to another person that has used both flux core and a gas feed that would use s flux core.

In a pinch? Sure, fine. Planning on installing multiple full factory panels and frame rails? I wouldn’t wish that on my worst enemy.
On a cheap AC welder, flux is trash, but not due to itself... It is what it is. On DC, not too bad, much better. But I will use flux, it has its uses, same as gas. Actually flux will penetrate deeper, and is a stronger weld.