First off, if you have rust, you've got to neutralize it. That means getting the paint off of it however you want. The starter set from POR-15 has a cleaner, that is awesome and water soluble, that's the first step, second is a product called "Metal Ready" it's a neutralizer for rust and a metal prep solution, and it too is water soluble, that's the second step. Third is the paint itself, what I have done on badly rusted parts is wire wheel the living daylights out of it to remove as much rust as possible, the other parts that only had light rust, I did the 3-step process over the old paint, but I painted the whole part so it was totally sealed with the POR-15 paint. Do I like this stuff, well, my whole car and all the parts will be painted, to quote an old commercial, over, under, around, and through, with the RestoMotive products, they're the maker of POR-15. HTH.
Ron