Note to Readers: I recently got a very interesting letter from a reader who described an alternative technique he uses for welding patch panels. I'd be very interested in hearing from other readers who may have used this technique of skip welding, so we can learn if the technique can be safely used in all cases or if there may be some potential problems from using this approach.
I'm writing in response to a question you addressed in your column in the April 2007 issue of SRM. The writer asked about the benefits of fully welding a seam versus skip welding. May I add some time-tested experience to the mix?
On spliced-in repair sections of outer panels, I have done many using both methods. When weighing the overall benefit of welding a seam completely, considering the warping and the resultant metal straightening necessary, I have found skip welding is a very good technique to use. In keeping an eye on many of these repairs for upward of 20 years, I have not had a single seam that became visible!
While I agree that a TIG-welded continuous seam is the benchmark, I am reminded of a boss I had many years ago who said, "Listen to the job and it will tell you what to do." There are jobs for which my TIG welder is not my first choice, and for which a continuous weld is not either. Often, a MIG machine set up correctly (with all parameters well thought out beforehand) will produce a joined panel with zero or near-zero heat distortion, which requires minimal metalworking and finishing. Since many of these parts have limited access to the backside, a minimal amount of filler may be necessary for finishing.
Fabricating a panel off the vehicle has the advantage of providing access to both sides. However, oftentimes a person may only need to replace the bottom 3 inches of a doorskin or quarter-panel, and may not have the tools, skills, or the access to the backside required to straighten the distortion caused by the heat of welding a continuous seam.
The method I use in those instances is to fabricate the repair section to be an accurate fit, and then clean both the original panel and the repair panel thoroughly. I prefer to underlap the repair panel by 1/4- to 1/2-inch, since this adds a heat sink and improves seam strength. In addition, that 1/4-inch allows a "slip plane" that I can use to modify the crown of the panel. I block up the area for a higher crown, or depress the area for less. On a long seam, I use very small screws to lock the seam together at the correct profile. I pre-coat both mating surfaces with a zinc-rich spray. Make sure there is zero zinc coating at the weld seam-you just can't weld on it. Scrape, scratch, or wire-brush it off completely. As you weld, this material migrates (as it should) into your weld area, so you must constantly clean, weld, clean, etc. I spend some practice time with the MIG on scrap material the same thickness as what I'll be welding. I'm looking for a "sweet spot" of the settings for heat and wire speed that will give me a quick, hot, flat weld nugget.