First of all, I am not selling POR-15. I just thought it was cool to get asked to be on one of their ad compaigns when we actually just called them to get instruction about how to use product on our new cobra project.
Anyway.. The company says that you can apply product directly on rusty metal, as long as you prep the metal. In other words, clean with a water based degreasor.
On our GT/CS, we media blasted the entire car. For those areas that we not going to get painted (by body color) we painted with POR 15. This included our engine bay, under side, truck floor, interior roof, behind dash, etc. We wanted to get something on the car as quickly as possible so rust would not have a chance to come back after the media blast.
Our next step was to cut out those areas and replace like floor boards and main frame rails. However, we learned a very valuable lesson in that POR 15 does not go away very easy and the welder does not like it. That was a struggle to weld through.
Like I said, I am not selling it, but did think it was a great product and we continue to use it on our new project cars.
Paul