What's the Best Way to Prep Unknown Condition Steel?

I've been using Ospho lately with pretty good results. I think it's a form of phosphoric acid. It's cheap, available locally, and easy to use. Doesn't hold on vertical surfaces so well but I'm going to try soaking a blue paper towel in it and wrapping it around and see how that works. When it is dry you can prime and paint over it. Eastwood has a bunch of options but they start to get expensive and the shipping isn't cheap unless you get a big order in.

Not positive but I think you can use epoxy primer over POR15 if you rough it up first.