I don't completely understand which spot you mean, but after the blasting during my rust hunting I didn't find nasty surprises anymore.
Nope, Eric,
That's unfortunately not completely true.
As you know our cars are unibody-constructed cars (or integral-body cars) and as such a lot of hollow beams like rockers. cross-members, etc. do provide the structural rigidity to the whole body structure.
When or if you sandblast the whole car -
as you (and I in my case) did - then you only remove the rust visible from outside.
However - as you also know - unibody structures do rust from inside - out
and that means that at least some of our hollow structures of the body do need special attention.
I do not want to terrify you now- but you do also understand that if you now looked into your hollow rail member structures with a special camera (good body shops do have that) - I am confident you would find rust !
That's not the end of the world, your car is still okay than (and your overall car body does look good from the pictures - it does!)
but nevertheless - prepare yourself to find some rust.
Especially prone for rust is the cavity behind the rear valance (that's where you just work on), the front cross member below the front grille section
and mostly
the area where the rear ends of the both main outer rails meet the rear longitudinal hollow members that go over the rear axle and this is also the area where the main crossmember behind the seats meets the rails.
That is the section indicated by your picture "project970.jpg",
the one where you put only a little patch on - this is the area John K is referring to.
Even though the whole region is really looking good on your car,
there _is_ some rust inside for sure and that's where you put the little patch on.
Well, you don't need to make a big construction site out of this now,
but be aware there _is_ quite some rust behind - at least on the inner surfaces.
That means that you need to apply lots of cavity wax into these boxed areas afterwards - very, very, very important!
Don't worry & keep on the good work !
Best,
Achim
(rust-dust inhaler)