Use a plasma cutter to quickly get the bulk of the old metal out of the way. This isn’t totally necessary, but makes things a little easier when you start separating the panels at the flanges.
To make the repair as clean as factory installation, we separate the panels at the seams. This process begins by locating all the spot welds and drilling pilot holes with a small drill bit.
Drilling spot welds is made easier and cleaner if you use a spot weld drilling bit like this. The bit lets you drill out the spot weld without opening up a large hole in both panels.
It’s probably unfair for us to jump ahead to this photo because it makes the process of panel separation look so easy. Nevertheless, this is what you’re aiming for–bare flanges ready to accept new panels. A little prying persuasion is generally necessary to separate the panels after the spot welds are drilled out, so the flanges will typically have to be straightened out with a hammer and dolly. We should also note that the panel is not just attached around the perimeter–notice the support brace in the center.
Use a weld-through primer (phosphate-rich) to cover the bare metal flanges before trial fitting the new panels. The primer will help prevent corrosion within the seam once the panels are welded together.
Next the new door sill panel is trial fit in place. Note that we also have the replacement cab corner mocked in place at this point. Since the two panels join together, it only makes sense to make sure they fit together properly before beginning to weld either one.
With proper fit achieved, we hold the new sill panels in place using Cleco-style clamps. You can see we’ve strayed slightly from OEM specs by placing the new sill panels on the outside of the upper flanges, rather than slipping them behind the flanges. This will simplify the repair and make the finished product tidier by keeping all the welds on the back of the panels.
A look at the rear of this sill shows how it fits together with the new cab corner. Note how the rocker panel portion of the sill panel overlaps the cab corner where they join. This seam was spot-welded from the factory, but we eventually welded the seam shut and metalfinished it for a cleaner appearance.
Now it’s finally time to begin welding. We used a TIG for most of our work, although a MIG machine would work fine for a repair like this. As mentioned before, the sill panels were primarily plug welded on the back side using the drilled-out spot weld holed, but we also had to fill a few Cleco holes on the top sides. (All seams were coated with seam sealer on the back.) Note that this cab corner has been tack welded in place at this point, and the seam between it and the sill panel is already welded and ground smooth.
The panels look as good as new originals — even better because most of the welding was done on the back of the panles, hiding the welds and eliminating a lot of finish work.