Check out the recent thread on Lord Fusor...
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=877414#Post877414 Basically the strength of the joint in using the adhesives is the overlap of the panels, so most repairs are installed using a stepped flanging process to accomodate the overlap but still keep the outer edges of the panels flush. The main detractor to using this method is the appearance of ghost lines at the seam in the final finish, as stated in the other thread, and/or delamination of the joint from improper cleaning or application methods.
What I normally suggest to people with welders, is to practice with scrap pieces to get the welder settings dialed in for a full penetration weld when butt welding, using the same thickness metal as you will be working on. Then once you feel comfortable with that, use the floor pans as your "practice area" to butt weld in the repair panels so that when you do get to working on the outside panels where it is more critical, you will feel more comfortable in tackling the repairs there.
Where some may suggest to use flanging on floor pan welding as it is easier, or nobody will see it, or various other reasons, bottom line is it makes for a repair subject to water intrusion and thus future rust. More importantly, flanging for either method, welding or adhesives, opens up the vehicle to scrutiny of prospective purchasers in the future that will see the flanged repair as substandard resulting in a possible adverse effect on the price. Where adhesives may serve as a viable option, any repair panel that does not appear to look as factory installed will be more likely to open up this scrutiny.
Here is a good read on welding in patch panels that may help you build some "book experience" to gain the confidence to use the welder for your repairs..
http://www.garagejournal.com/forum/showthread.php?t=53534
