I don't use POR so I cant say for sure, but when I get things like silicon contamination, oil residue that has soaked into the metal, things that are really almost impossible, I use PPG's DX330 cleaner. Its a Naptha based cleaner that makes it so that paint sticks very well. Maybe after using 34 grit sanding disks and taking off the top metal substrate, then cleaning with a good cleaner? If you don't have DX330, try lacquer thinner to start. All this amounts to is a challenge. You will overcome it.
