There maybe better paint but I used Rustolium from home depot and holding up well. Can't remember if I used their primer first or not. I do keep trailer in a garage.
For heavier duty.. Ronan makes some really tough water based paint. I use it on billboard structures (so does CBS and Lamar) and painted my flat bed trailer and it's tough and easy to use . You can brush it or put in a sprayer. They have a primer too but unless the rust is really bad and deep you don't need it. If you use white you will have to use the primer with stain block in it. I will use Ronan next time for sure. They can ups it to you.