Removal and replacement of welded body panels
The complete bodyshell of most cars is built up from a variety of sections which are resistance welded together. This type of weld is more commonly known as a "spot weld" due to its' appearance - a circular weld about 1/4" wide. Each section of a modern car contributes to the strength of the whole shell, so it becomes vital that if any of those sections are removed, a replacement must be welded securely back into position, using a similar number of welds in the same places as the originals, to restore rigidity.
One noticeable trend of the last few years is the move towards larger and fewer panels. An example of this is a normal family saloon body side. Previously the side view (minus doors etc. ) would be built up using sills, roof rails, door pillars and rear quarter, plus other smaller sections where needed. Now a large proportion of vehicles would have this full bodyside produced in one large pressing, so removing a lot of welded joints at one stroke. The downside of this progress is that some panels are available only as large sections, or need to be joined with precision mid-panel.
Obviously different makes and models will have design and detail differences, but the general principles are the same for most cars. Our examples include a simple front wing change, followed by a more ambitious rear quarter\rear wing replacement for those with more courage.
Removing a welded front wing
Remove trim, bumpers etc. as required. It can make the job easier in some cases if the bulk of the old panel is cut off first with a sharp chisel, leaving the welded areas much more accessible. Make a quick note of where and how it's welded, so that the new panel can be welded in the same places. (Note: some wings may be welded on to raised mounts, making weld position important!) Now each spot weld needs to be located and cleanly removed. Purpose-made spotweld drills are available for this, which work on the basic principle of a hole saw, cutting around the weld button to separate the two panels. Where access is limited, a sharp chisel can be used to carefully cut the outer panel only, leaving the bulk of the spotweld still attached to the underlying panel. It is much safer to leave excess metal on the supporting panels, as this can be cleaned away later- much better than not leaving any metal to weld too!
Whichever method you choose, the most important part is not to cut away the supporting panel or flanges that you will need to weld the new wing to. With the bulk of the old panel removed, any remaining bits of panel or spot weld studs can be easily ground away with an angle grinder to leave a smooth, clean surface.
With all mounting edges clean, check for damaged\twisted areas caused during the removal operation. Once you are satisfied with the results, try the new panel in place for fit. Genuine manufacturers' panels and most reputable aftermarket panels should be fairly easy to align, but there are still poorly fitting panels available, so it's buyer beware. Ensure the panel is capable of fitting properly before removing it again for the next stage
Replacing a welded front wing
Clean all paint and primer from the areas to be welded. For those with a spot-welding attachment for their welding equipment, skip the next step. If not, drill 1/4" holes in the panel mounting points to correspond with the intended new spot welds.
Refit the new panel to the car, double-checking its' alignment with door edges, bonnet , headlamp surrounds and anywhere else it has to fit. Clamp it in place with vice-grips, small g-clamps or whatever fastening method you are using. In awkward corners a small self-tapping screw might be easier, as you must allow for constant checking of bonnet gaps etc., before the panel is finally welded into position.
If using a spot-weld attachment, follow the makers instructions for power and timer settings.
Using a standard MIG welder, set at a moderately low setting, start welding each "button weld" from the lower metal surface, building up a puddle of molten metal which flows into the edges of the hole drilled in the new panel. A little bit of practice with this method can produce consistent welds of good quality and appearance. Don't forget to weld under the front panel or down the inside of the door pillar seam if the original panel was welded there. Appearance isn't quite as important for these areas as long as a good strong joint is made.
When all joints have been welded, they can be tidied up if need be using an angle grinder with a stone disc, with final finishing, etch priming etc., being carried out as normal. A little bit of preparation using stopper over the visible spot welds under the bonnet will produce a smooth finish virtually indistinguishable from factory welds.