^ This. When I find a bit of rust under the truck, it gets a quick wire brush, then rusty metal primer and black paint. The rusty metal primer has acids in it that supposedly halt rust and bond very well to it, based on my experience using them the last ~10 years.
Do note "rust reformer" is for things so rusted they're almost beyond saving. The "rusty metal" primer is likely the one most of you want.
What's the difference between Rust-Oleum's metal primer and reformer? A full explanation and comparison of these two products.
azrust.com