Is it not possible to buy something similar to the stuff they use at the factory, the thick rubbery stuff. I've never managed to find anything like it in the shops. The Waxy type stuff isn't up to much in my experience, just seems to trap the moisture and hide the rust that continues underneath. And it doesn't seem to have any anti corrosion chemicals in it as far as I can make out, certainly doesn't have the affect rust killers have such as Kurust.
I was going to use rust killer followed by Hammerite (good stuff) possibly followed by some of the black waxy goop (but only when sure the rust underneath was completely treated and gone).