Back when I used to work in the body shop, we used an acid that we got at the body supply store, to spray over sand blasted areas that had pitting in it. You sprayed it on and let it set for about 5-10min. You had to keep it wet during this time. Meaning keep spraying it. After ten minutes, you rinsed it with water and blew it dry. It left a white film and you left that film on the panel, and shot primer over it. The film helped the primer adhere to the panel. We used that stuff a lot and it worked great.

Roger, Matty might be talking about a "wash primer", that is either yellow or green that they spray over areas that were bare metal to protect them from rusting. There are also other sealer primers that some guys spray an entire coat or two and then resand that before paint. And top it with the wash primer. I haven't been in it a while so I'm sure it's changed.