Hammer Paint Over Rust
In general, paints like this do indeed stick to the rust just fine and encapsulate it by linking together and forming a hard surface. This is good if you want something to look good, but it doesn't do much to stop the rusting process. Eventually, the metal below the paint will rust further, and in the process expand. This will cause a crack in the paint, and eventually it will chip off. If the steel angle in question isn't structurally important, that might be fine. Depending on your climate, painting over it might buy you a few months or a few years. If failure of the steel would be a big problem, then you should sandblast or otherwise get to white metal, and pick a coating system designed for rust prevention.