Some pictorials to help explain what is occurring, may be a good lesson for someone else trying this in the future....
Any time we weld, especially with a MIG, you have a shrink that occurs circumferentially about the weld dot, pulling from ALL directions. Despite efforts to allow cooling in between, the majority of the shrink and distortion in using the MIG is the starting and stopping. If one were able to tack the pieces in place and then perform a non-stop weld from one end to the other (using TIG or Gas) you would see much less distortion as the panel heats up gradually as the weld puddle progresses across the panel, and cools down gradually in the same fashion. With a MIG, even welding one dot at a time, it is a quick in and out, for the possibility of more distortion from shrinking. Planishing with either method is a must to counter the effects of shrinking, but a continuous pass will help to limit the
weld distortion.
Looking at the running board:
http://i5.photobucket.com/albums/y167/rmccartney/Metalworking/RunningBoard.jpgThe upward bent flange near the body helps to keep things from moving. Ideally, if a flange were bent on the outside in a similar fashion, it would have eliminated much of the distortion you realized, depending on the heat introduced when welding on the brackets beneath. Given that welds were used in attaching the outside flange, with each weld dot we realize a shrink at each spot, and the distortion that comes with it. As the weld cools it is pulling the metal in from all directions, even though the picture only shows two directions. The shrink/pull in the direction of the red arrows may tend to draw downward, losing some of the sharp 90 where there may be more of a slight radius at the corner. Anything pulling in the direction of the blue arrows is going to shrink the length along the edge. So when you combine an area that doesn't want to move (yellow line) and an area that is moving (blue arrows), that transition area in the middle starts to form a sinusoidal wave as the shrink causes what outwardly appears as extra metal.. One of the best samples of this, lay a piece of paper in front of you landscape. Left index finger on lower left corner, right index finger on lower right corner, and push your fingers toward each other, representing weld shrinkage. Note the buckle in the lower edge of the paper. This is what is happening to your running boards.
As with any oil can, the "fix" is determined by the cause. Here we have shrinking that occurred from welding, so the CORRECT manner of fixing this type of oil can is to stretch the weld and HAZ. Not an easy task given the location. But likely the easiest way to do so, given the 90* bend, is to use a section of press brake die with a sharp 90, clamp in a bench vise and hammer toward the top of the die, parallel with the 90* planes, where we would expect the weld and HAZ area to be. I would note that welds, especially if MIG was used, will be notably HARD. You would be wise to insure the press brake die is hardened or make it so using Casenite or other hardening process.
http://i5.photobucket.com/albums/y167/rmccartney/Metalworking/Planishing1.jpghttp://i5.photobucket.com/albums/y167/rmccartney/Metalworking/Planishing2.jpgNow although this is the correct method of repair, given the difficulty of weld location and other factors, the REALISTIC method of repair, after exhausting planishing efforts, will be to shrink the bulges. Heating with a torch, I would heat to a size of about a dime, and cool. Start in the center of each "bulge" and work your way out circumferentially.
FOOD FOR THOUGHT. You've done some nice fabrication work on your truck, love the dually. But welding and flat sheet tend to cause issues like you have here. To help combat this in a fabrication process, bending both the inner and outer flanges would eliminate heat introduced distortion. Adding ribs using a Pullmax or other similar reciprocating machine will make the panel more resistant to distortion with the introduction of heat. A bead roller may work, but it shouldn't have HF or Princess Auto on the box if you're going to tackle 16 ga steel. Hope my ramblings help out..
