I'm not to familiar with flint but i do know that if the blast profile is not to deep river sand will give you a good even blast. I've used river sand on stainless on many projects and haven't had a failure yet!
Ther are many coatings that will work but the first thing you need to do is look under that shop primer. I'm willing to bet that the primer was applied over mil scale. If i'm right it doesn't matter what top coats you use they will all fail if you do not blast off the mil scale.
