I've used Rustoleum on a thousand different things over the years and wouldn't hesitate to use it on anything metal. It's never lifted, flaked, checked or failed to adhere. Surface preparation is key of course.

 

This is what I used on my rear inner wheelwells in the trunk and on my my floorpans when I bought my car in 1996. Have not had any rust issues at all. This stuff is buttit proof. I highly recommend it. See attached PDF file:

http://tds.loctite.com/tds5/docs/EXTRT-EN.PDF

 

Nothing wrong with your plan. Use the red Rustoleum Rusty Metal Primer. It is designed to adhere to a rusted surface. You have to top coat with something else. Primer will not stop moisture. I used Rustoleum 7777 Satin Black to top coat. This process will not convert the rust but it will seal it. Rust needs moisture to continue spreading. As long as it is not actually coming through from the back of the metal you should be fine. I also have used the rust converters and they would do the job for you as well. As long as you seal the rust in and keep the moisture out it should not get worse on you.

 

you guys are awesome, tons of great info... thanks all.