The product you need is phosphoric acid. All of the rust converter products have it. It turns rust into a stable, black compound called iron phosphate. The excess product is sort of a blue-white compound that should be gently removed before painting or the paint won't stick. I often use a stiff brush or a rotary abrasive pad found in automotive and body shop stores. (Not sandpaper, but more of a scrubbing pad).
I have not had much luck with the spray-on products, since they just don't apply enough material. I usually use the brush-on product, which is water-soluble and quick-drying. Lots of products are readily available at automotive stores and big-box stores such as Wal-Mart and Home Depot.
The converted rust must be coated with a primer and paint.
Edit: I just looked at the Amazon reviews for POR-15 and it looks intriguing. Interesting that you have to etch clean steel or the product won't stick well.