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

I have been using ospho for years. If the metal is pitted bad you might have to spot blast the bad areas to get it clean.
I use steel wool with the ospho and scrub the metal with it. After scrubbing wipe it dry. It will dry after an hour or so and will stay protected from flash rusting for a very long time as long as it doesnt get wet. When your ready to prime it should be sanded down to remove the coating and rough up the metal for adhesion.