68' Dart

I don't know if you can see it but where I was sanding, the epoxy primer came off super easy and there was a bunch of tiny black spots under it. Doesn't appear to be rust but it takes sanding down to bare metal to remove them. The rest of the car was ok. The car was soda blasted a couple of years ago, washed, dryed with compressed air, wiped down and primered. Could this be soda residue that didn't get cleaned very good?