Jim, I would suggest to practice up a bit on panel fitment and use a butt weld.. In the lap vs. butt weld debate, regardless of whether one side of the joint has one, two, three thicknesses or more, the heat from welding will cause shrinking and the resulting distortion from this shrinking of the panel adjacent to the weld. But to clarify, the greater the thickness and mass, the more heat required for the weld. Thus, the lap joint actually takes more heat to perform the weld than a butt joint. What you must consider in your methods is how on earth you will fix these issues to return the panel after welding to the original shape that was there before. The plain and simple of it being, that a single sheet on either side of a butt weld will be easiest to planish to remove this shrinkage distortion. Two thickness will be near impossible to planish without inflicting more damage.
Something I've emphasized many times is the importance of consistency. This starts from the fitment of the panel, to the methods used in welding, to the methods used in planishing and grinding welds. If you have consistency in all of your processes, the end result will save you work in the long run. I'll discuss the various welding methods, as discussing the differences will help to better understand consistency:
Gas (O/A) or Tig would be a cleaner choice in welding in that they leave no splatter or slag all over the place. The amount of warpage is relative to the amount of heat you put into the panel via how long you sit there, so typically, MIG vs. TIG (when applying filler rod with the TIG), the MIG would have less of a HAZ. However, if you can trim your joints to zero gaps such that a no-filler weld can be performed (fusion welding) in using the TIG or O/A, then you should have about as small a HAZ as possible, and as consistent in width as possible, for less distortion. To explain this further, various starting and stopping in your weld will cause inconsistent width of the HAZ, along with the shrinking effects that come with it. This may lead to a "wavy" distortion. The process with the most consistent heat and consistent speed, would be the fusion weld (no filler), using TIG or Gas, and running from one end to the other without stopping. Performing the weld in this fashion will help to keep the HAZ, and all the other conditional reactions consistent, for less distortion.
Here is a excerpt from David Gardiner's instructional video, He is a coachbuilder in England that primarily uses gas welding. He says that his method uses little to no filler, uses a faster process, for a narrower more consistent width HAZ, less distortion, and little if any cleanup as far as grinding welds. I'd also recommend the purchase of his video, he does a nice job of demonstrating fabrication techniques using little more than hand tools.
Having said all that, most hobbyists have nothing more than a small MIG in their home shop, and those conditions are hard to duplicate with the MIG when welding sheet metal. Even though it is not compatible with a full pass weld when dealing with sheetmetal, you can still apply the same principles of consistency in using a MIG and "dot" welding. Single dots, skip around, use same overlap, same size dots (elapsed time of trigger pull) etc. This won't make the weld as nice as the fusion weld described above, but it will help with consistency if the MIG is what you're using. The biggest thing on consistency with the MIG is to practice on some scrap pieces the same guage as you'll be working with to insure your welder is set up correctly for a full penetration weld. It's hard to be consistent if you're putting in the patches and still fiddling with welder settings, (practicing on the good stuff).
Next, if you aren't very good at trimming the panels to fit tightly together, the O/A or TIG is going to be more likely to blow holes (depending on your proficiency to keep up in adding filler rod where needed); this is an area where the MIG is more forgiving as it is automatically feeding filler. But again, more filler is creating more heat, more shrinking. So any inconsistent panel fitment, i.e.: tight joint in some spots and wide gaps in others in the same panel, will result in more filler, more heat, and more shrinkage in those wider gaps, for inconsistent shrinking results and likely a wavy panel will result. The O/A and TIG processes prefer a tight joint for less chance of blowout. Lastly, TIG can sometimes be awkward to use in some remote locations (i.e.: under a car, any hard to reach location where using a foot pedal is cumbersome) O/A will be a bit easier as no foot pedal, and the MIG is more of a point and shoot type deal. All things to consider when chosing your methods.
Now, back to the panel flanger. I don't personally use panel adhesive, but this is actually one of the few areas where the flanger should be used. But the downside, regardless of attachment method in using a flanger, is you'll have two panels on one side of the "joint", one on the other, and despite a skim of filler over top to finish it off, you will more than likely see a ghost reference line in the finished paint as one side has a differing expansion rate due to the increased thickness. Panel adhesive application will also require plenty of clamps to keep everything snug while the adhesive sets. Mainly used on newer repairs in the collision industry, much of their driving factor is getting something in and out the door to keep costs down to appease the insurance companies. I can't say if it would be a viable option for you as I haven't used it. But on the other hand, I'm sure if you asked the local body shop's opinion, they would rave about it, as less time = more money in their case. This may bias their opinion for that reason.
In the end, you need to pick the method that works best for you, and produces results you will be happy with. I'm sure I may have created as many questions as answers, but hopefully it will help expand your research for a more informed decision.
