well, found a good link for all you ever wanted to know about rust....great site. Called a rust primer, not a product but a study.

Here is a link that might be useful: Rust Primer

Anything I want to keep looking rusty (but not continuing to rust), I spray with a clear spray. I don't like the shiny look so I don't use polyurathane. Sometimes I use walmart spray and sometime a more expensive brand. Usually work for me....