Longer lasting repairs to rusted panels
One material which has been in use on the production line and in repair shops since cars first appeared is body solder.
Body solder differs from conventional solders in that it becomes semi-solid, i.e., spreadable, before it melts fully into liquid. A small gas blowlamp is ideal for soldering small to moderate areas. It can only be applied properly to absolutely clean bare metal, using a liquid or paste flux. Always observe sensible precautions; remove any flammable materials or plastics from the surrounding area, and check the rear of the repair for wires, pipes or other items which may be damaged or catch fire.
Grind the rusted area back to bare metal, finishing with a wire brush if necessary to remove all traces of paint and rust. Tap the holed metal so that it is just below the normal panel surface.
The area must be "tinned" first by applying a flux/solder mix to the heated area then immediately wiping excess solder away with a clean cloth to leave a smooth, solder coated surface. A build up of body solder can then be melted into the repair, and with careful use of heat and a small wooden paddle, spread across the area to be filled. Just enough heat must be applied to the panel to ensure the solder flows properly into the tinned area, without allowing too much heat to build up and warp the panel. Practice makes perfect, and horizontal repairs will be much easier than any on a vertical surface.
When cooled, clean away any surplus flux before grinding roughly to shape and finishing with filler as normal. Avoid breathing grinding dust, as solder contains lead.
Methods for patching rust holes in bodywork
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.