Mustang, here is the story that was given to me. In retrospect, it makes sense. The metal goes through the media blast process. It opens the pores of the metal. The metal will corode quickly, but as well, anytime you touch it or the metal comes in contact with oil, that oil will inbed itself into the open pores. So, what to do? If it were mine (the great qualification) befor I would ever put any primer on I would use a liberal dose of good quality degreaser. This will not be easy as I remember the surface is rather ruff to the feel.
Remember a degreaser works quickly, its wipe on, wipe off. The degreaser "foats" the oils where they can be wiped away. So, liberal amount, then wipe quickly. The degreaser will then dry on its own. I would probably apply degreaser 2-3 times to make certain that one would get all of the grease off the surface. If not, 6 months from the final paint job, while the car is sitting out in the sun enjoying a wonderful car show, the oils left in the metal will decide it needs to see the surface. Thus, the dreaded fish eye or worse.
Now, that from a guy who has done this only a few times and is really parinoid about grease on a to be painted surface. We went three days after our media blast and got scared.
Brian