You need to use a stiff wire brush to get off all the loose rust. Also the surface needs to be clean and dry - so wipe it down after you wire brush.

I would first use something like Rustoleum Rust Reformer. It converts the iron oxide (rust) so it won't continue to rust under the top coat.

Then spray with the Rustoleum of your choice.

Geoffrey, thank you very much for your suggestion, which is very helpful.