Most enthusiasts don't like to waste parts or pay for something they don't need, so the idea of buying a replacement sheet metal part with the intention of only using a portion of it may feel counterintuitive. Still, that's often the best way to proceed to achieve the most correct restoration. Fortunately, we have more reproduction sheet metal available than ever before, and the more accurate our repair panel is, more likely it is that everything will fit together like it is supposed to. Also, a less invasive repair means that more of the original car is preserved.
Muscle Car Restorations in Chippewa Falls, WI deals with situations like this on a regular basis. Recently, they needed to repair an outside seam of a lower cowl panel on a 1970 Nova. Replacing the entire cowl side would have involved cutting out a lot more of the car than was necessary to make the repair, so they simply used only the portion of the replacement panel. This not only saved time, it also preserved a lot of original spot welds and an original factory stamping that was not on the reproduction part. Follow the process to see how a repar panel is sectioned out and fitted to the car.
This outside cowl seam is rusted through and needs to be replaced. There are really only two panels that need to be replaced to fix this.
The cowl side lower panel as purchased includes the entire “box” but MCR only needs a portion of it to make the repair.
When possible, it is best remove panels at their seams by drilling out the factory spot welds.
One of the challenges with repairing unibody structures can be gaining access to the multitude of spot welds holding everything together. Since the outer portion of this panel is being discarded anyway, it was cut out just to make room to break the spot welds. Always drill the spots welds from the side that is being replaced, so the side that is being kept will retain its factory appearance.
There is one corner where there is no seam that can be separated, so the metal will have to be cut here to finish removing the part.
This is the section that will be replaced. The seam under the right hand is where the rust damage has occurred.
The same spot welds are drilled on the new panel to separate the part that will be used for the repair.
Like the original on the car, a cut needs to be made here to remove the unneeded portion. The cut is made long so the new panel will overlap the old a bit.
The portion in the foreground is what will be used. The rest, in this case, can be saved for a potential future repair on another car or scrapped.
Here’s a view of the portion that will be left on the car. The gold paint is Copperweld Weld-Through Coating, which is used to prevent rust from reforming inside the welded seams.
While spot welds are preferred, plug welding the new panel in place is a perfectly acceptable alternative.
With the new panel mostly welded in place, a cut can now be made, using the edge of the original cut as a guide to trim the new part, leaving a perfectly sized gap to seamlessly butt weld the two panels together.
The approximately 1/16th-inch gap left by the cutting wheel is the perfect width needed to butt weld sheet metal parts. Start by placing a tack weld every 1 or 2 inches along the entire seam, making sure to keep both parts perfectly aligned. Then, using 4 or 5 quick tack welds in a row, move about the seam closing up the gap. Be sure to air quench after each tack weld to insure against any heat warping.
Once the entire gap is tack welded together, the welds are then ground smooth to produce a seamless repair. Keep in mind that 20-gauge sheet metal is only 0.0375-inch thick so it’s critical that only the welds are ground flush and that none of the surrounding metal be ground thin.
Here is the finished repair. Notice the stamped numbers to the left of the fresh air intake opening that would not have been retained if the whole part had been used.