sandblasting doesn't get ALL the rust! This is my inner fender that was sandblasted in 2002 and painted with epoxy primer and replaced on the car.
As you can see the areas that were sandblasted and epoxied are fine but .... the rust continued from between the panels
So I drilled out the spot welds and cleaned everything and I will put some POR15 on the areas tomorrow when I can get the time for two coats and the final black. Then I'll weld her up and all is good with this one, the other one needs a patch fabricated and the same steps to stop the rust.
Lesson learned ... do it right the first time.