I've used POR15 and Eastwood's Rust Encapsulator on different vehicles.  Had so-so results with the POR15 and the Rust Encapsulator the verdict is still out on.  POR15 fades if you leave it exposed to sunlight and I had problems with it shrinking back around the edges in places.
The Rust Encapsulator I only recently used on some wheels and a lift for my Ranger and neither have done anything but sit on the floor since I painted them.  The Rust Encapsulator seems to be a good primer though, I painted the wheels with Rustoleum Hammered Black afterwards and it seems to have stuck to them well.

I think next I'll try the Chassis Saver like Quaddawg used.  I've got rust in the same places keith has and more.  I don't expect this thing to run forever but I'd like it to get me through a few more winters.

POR15, Rust Encapsulator, and Chassis Saver can all be painted over, you just have to follow their instructions and do it soon after application for best results.

Loctite also makes a "rust converter" product called "Extend".  Clean the surface of dirt, spray or paint the product on and the rust is converted or absorbed by the product.

They also have a product called "Rust Dissolver" that strips steel down to the bare metal eliminating sanding or scraping.

Yep, links to the Loctite web page describing these products (with part numbers) can be found by searching for loctite at Ack's FAQ (link is in my Signature)  select the specialty products link.