I was under the impression that POR-15 also changed rust into an inert material, that way you could just clean the rust and apply it right over without using a rust converter first.
Check this tech article, it says the same thing (might not make it true tho )(/* )
http://moparmusclemagazine.com/techarticles/mopp_0506_panel_replace/index1.html
Check this tech article, it says the same thing (might not make it true tho )(/* )
http://moparmusclemagazine.com/techarticles/mopp_0506_panel_replace/index1.html