It is sometimes viable to repair rusted sections by removing the rusted area and replacing it with a steel patch welded or brazed into place. The same precautions regarding heat etc., apply as for using body solder, plus extra care with heat build-up warping the rest of the panel. Use of a MIG welder is preferable, with oxy-acetylene brazing or welding not really recommended.
Cut away the rusted section as carefully as possible to minimize further damage. A small cutting disk in an angle grinder or power jigsaw can be used if care is taken to check no damage is being done to other panels, etc. If possible form a flange on the cut edges of the panel, to create a lowered step to drop a repair section in to. The repair section should be cut and shaped so that it fits as perfectly as possible into the recess created. Clean up both sides of the original panel and the repair section before beginning to weld into place, especially if using a MIG welding machine. Wherever possible weld the patch into place completely without any gaps, which would allow moisture to gain access again. Weld a short section at a time, possibly an inch or two, allowing the repair to cool before starting again. This will reduce warping to a minimum. If a continuous seam weld is not possible or practical, body solder applied properly to the remaining joints will produce a far longer lasting repair. Whichever method is chosen, it is good practice to seal and rustproof the back of the repaired area. Final finishing is as normal, using bodyfiller etc., to blend the patch into the existing bodywork.
