POR15 is the go-to rust stabilization and sealing paint for the classic car guys (https://por15.com/collections/rust-prevention). It chemically binds to both metal and rust with an epoxy-like coating. I used it on several cars back in the days when I was hot-rodding 60s and 70s muscle cars. Works great.
My understanding is that Hammerite is not formulated for automotive use conditions. POR15 specifically is. I haven't used the Bilt Hamber products, so I can't speak to those.