Corroded Trunk Seams

So I'm not sure what to make of this.

The dart turned out really well after soda blasting, but the trunk seams all have rust in them. This one is the worst, although the two under the trunk hinges are just as bad. Access is going to be a ***** cause the hinge support piece is also spot-welded at that area.

Anyone have any thoughts as to what caused this? All the seams were coated with white seam sealer (factory) but had started to flake off where the rust is.

And if you have a parts-car 67-9 Dart that you'd cut the seams out, I'd pay for them.