I cut out and patch substantial rust damage on the unibody frame of my 95 Saturn. This is immediately behind one of the front wheels. The car was undercoated when it was new but the tires have sandblasted this section to expose the metal. Salt and water have rusted away the structure, which I had to cut out and replace.
This may not be a professionally approved method but I do feel that is it solid. I will be coating the inside of the frame with rust inhibitor.
Basically I cut away all the rust down to solid metal. Then I created a patch from scrap, formed it, and welded it solidly into place. Finally I painted it and will be undercoating it too.
Redneck repair? Perhaps it is!