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The paint is called POR-15, for Paint Over Rust. Great stuff. It forms an epoxy-like finish.
It will stain anything it touches permanently. Wear gloves and a respirator. It dries from contact with moisture, even in the air, so cans must be resealed tightly. Most users uses multi layers of plastic wrap on the top of the can before the lid is pressed back on.
I was talking about rust converter pain like Rustoleum.
FYI, to use properly, POR-15 must be applied to bare metal, so you would first have to strip the factory paint. And at $90/qt it's not cheap stuff. I wouldn't do it.
Here in the rust belt, I’ve tried ‘em all. The only disappointment I’ve had with rust converter is that sometimes it doesn’t seem to penetrate all rust layers, so a rust ghost comes back.
Yes, POR-15 is expensive, but I do get predictable results 99% of the time. As the name states, it is for painting over rust, not other paint.
OP, to get the best result, you would trim the loose rust away, apply POR to the rusted areas, and prime & paint the whole lower panel area whatever color you want.
Sometimes I have seen vans spray rubberized bedliner or undercoating directly on the rust and hope for a few seasons before the rust shows through again. This is probably why most vans have a black stripe rocker area.