you can't use any sealants pre-cat unfortunately, as they could make their way through the cat and damage it as they're burnt
if that's what's causing your prob, there's not much you can do about it easily - i did look into brazing the pitting and refinishing it (lots of work), or cutting off the mouth of the cat and welding another on (impractical for me, as the metalwork's v thick at that point and didn't want to risk bodging it and writing off the cat)
you couldn't be sure that this pitting's a probability in your case without looking at the cat though