Damian;

That's the downside of taking things down to baremetal when doing a restauration....you expose highly reactive virgin metal again, so you damnwell better passivate and coat it well (fortunately, I love blasting for the fact that it also gives the surface excellent "tooth" for holding on to new coatings), in order to keep mother nature from doing what she does (and very effectively)!

In your pic, I just see some surface rust from standing water...but that's where it starts!...its a hidden place, so I would just apply some rusty metal primer, followed by a top coat.

Cheers