For me, and this may be nonsense, but, when I was looking for painite, I found that, for the same ring on different spawns, there would either be 'roids with it, or there would not.
So basically if I didn't find any within the first, say, four or five rocks tested, I wouldn't find any for the rest of that session.
I guess what I'm saying is: if you come across a field that doesn't offer up painite at all, re-log, or jump out and back in again.
Of course, it *might* just be really rare and I was really unlucky a few hours straight, but there was a marked difference when I fell into a field that had painite 'roids. It was still like 1/4 or 1/6 rocks or so rare, but it wasn't 1/50 rare.