There is a continual interest by the owners of steel trailers, on how to prepare rust before a finish is installed. There are paint primers that can be used over rust, but unless they have ingredients to chemically stop the oxidation, the finish may fail.

I just received a catalog form the Eastwood company. http://www.eastwood.com/   1 800 345 1178. They have a full line of automotive tools and supplies for the restoration of automobiles. They are presently having a 25% off sale.

In addition to the urethane paints that they sell at prices lower than my auto parts store, they also sell a full line of rust converters. This is what is needed to stop the rust and prepare the surface for a primer coating.

In the aircraft and marine industry, a converter which is a phosphoric acid product called Ospho is commonly used  http://www.ospho.com/  If you pull up their web site they have instructions for its usage and application. Steel tubing that is treated internally before painting lasts significantly longer.

No I am not a lobbyist for these companies. I have used the products.

Follow the manufacturer's instructions

BOL  Gard