I am media blasting using this black sand (black beauty?) medium grit. I am blasting with 90psi on a spot blaster and trying to get rust out of surface rusted but heavily pitted metal. I blast it but when I go and use picklex (rust converter) I can still see black marks deep inside the pits! What am I doing wrong? Is black beauty too big to get in there and remove rusted metal? What can I do?