POR-15 paint
I used it on the floor pan of my GN. Worked pretty good I think. I had heavy surface rust, so I used a 4-1/2" angle grinder with a wire brush to knock off all the scale. Then prep'd with their marine clean and etch. The POR-15 brushed on smooth with a foam brush and it dried glass smooth. Nearly three years later, it is still rust free. If you use it on anything that is exposed to sunlight, you will need to paint over it with something that is UV resistant. Otherwise, it will fade.