When I refurbished my chassis I stripped off the remaining powdercoat with nitromors and repainted using the POR system and got a very good finish. The marine clean is vital as is the etch coat. This then gets washed in wate prior to the black top coat. I did have a problem in 2 or 3 areas where the top coat reacted with something, possibly unwashed etch or even traces of the nitromors, that required repainting. With an ordinary brush the POR top coat dries to a very hard shiny durable finish. Note it is not UV stable, so I used the alternative POR top coat on small areas exposed around the front suspension pick ups.
Alot of hard work compared to shot blasting and re -powdercoating, but I wasn't in a rush.