What is the proper method of fixing car rust?

The affected area is cut out , using a 1mm cutting disc ,including all steel that is pitted from the inside. While the area is open, any visible and reachable areas should be cleaned of surface rust, rust converter applied, followed by by an oil- based cold gavanising paint. A new piece of zinc-aneal plate is then fabricated to replace the rusted section, accurately enough to file finish or finish with a thin skim of body filler or lead. Cutting out the affected area in a single piece can help here, for use as a template, particularly where curves are involved. The edges may be butt- joined with a 1/2 - 1mm gap, to allow for expansion as the section is tack-welded into place. Tack in alternate locations until the tacks are about 1" apart. The section is then fully mig welded , in one inch stitches in alternate locations, allowing cooling in between to minimise distortion. Before filling , the welds are ground level. When the repair is completed, it should be treated on the inside with a wax-based anti-rust compound.

Where a rusted area includes a spot-welded seam, the spot-welds should be removed using a spot weld removing tool, or a 3/8" drill bit, the seam separated and stripped of rust. The new section should include a new seam, plug-welded at the same locations as the old spot-welds.

As an alternative to hand making a new section, rust repair sections are sometimes available from auto suppliers. Also, It is sometimes a better option to cut the required section from another vehicle.