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I wouldn't put por 15 over a different coating,not how it was designed to work. It is designed to be used over phosphoric acid prepped metal. Part of a 3 step process ( water based cleaner) marine clean ( phosphoric acid etch) metal ready prep ( top coat ) Por 15. I use a similiar 3 step process lots of people have for decades. Mine step 1 simple green or purple power cleaner. Ospho phosphoric etch. Top coat depends on where I'm using it and for what reason. POR 15 over a different coating maybe would be fine,but the price and work involved I'd use, POR15 by the book or whatever your previous coating manufacturer recommends.
