The last thing I would ever do with rust is to SEAL it. That's just giving it permission to continue to destroy your steel, and providing a nice cover so you don't see it happening.
That's why I don't like powder coating. It will eventually fail, and when it does, it conceals the damage until it's pretty far along. Stealth destruction.
Paint will fail sooner, but it won't lie to you and say everything is fine the way powder coating will. It will show the rust almost as soon as it starts, allowing you to clean it up and repaint before serious damage is done.
That said, if you can't get to it to clean it up, try Fluid Film. It will penetrate and stop the rust. Cheap, easy.
I have no idea if it's better than ospho, as I have not compared them. I just know it works well, is easy to use, and it's comparatively cheap.