Por 15 ????
I love it!
Just knock off the scale rust with a wire brush and paint the Por 15 over. I have taken suspension parts down to bare metal, hit them with the hose and left them outside until they got a nice covering of rust so I could use the Por 15 on them
Wear gloves and clothes that you don't care about. I use the 2 or 4 inch cheap throw away brushes to paint it on.