Sounds about like the stuff I used to use when cutting out rust from my old truck in HS, we'd use that along with some sheet metal and bondo to patch up the cab corners and rear wheel wells. Of course I didn't know any better at 17 and used aluminum pop rivets as I didn't have a wire welder. A few months later we had rust again, but it was due to the electrolysis between the aluminum rivets and steel. The areas where we used the rust converter were fine.
I'm surprised you didn't setup an electrolysis cleaning tank for those.
To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault