This is a cross-member supporting the nose of the rear differential.
The rust developed between the cross-member and the outer shell.
After cleaning I've used a rust converter from Eastwood.
I've made a pattern and fabricated a piece of metal to match the missing piece. I was tough to weld there; also the metal was thin and corroded inside.
After a coat of self-itching primer and 2 coats of paint.
I've used a brush-on sealer to make sure that water will not go back in there through the welds. This is a paintable sealer; it can be coated at a later date.
In the trunk, under the spare wheel. Here I have put a floor jack underneath to get the metal back on the same plane.
The green is not matching but the spare wheel will cover it.
After painting, I've used the same brush-on sealer to protect the welds.