Stuart,
I used Rust Mort when I restored my 63 Impala. It worked just great at neutralizing the rust to a black mass which I finished primed and painted over. That was 12 years ago and the rust hasn't reappeared. A word of caution - wear rubber gloves and don't spill it on any painted surfaces or contrete floor, because it will etch and stain it. (personal experience)

Recently, I purchased Neutra Rust from Canadian Tire and thought the results were every bit as good. It was a lot cheaper too. I got my Rust Mort from an autobody supply shop.