I've never used it for a small patch, but often have glued stuff in or replaced panels with it where I work. It is great for non structural peices and you aren't burning off corrosion protection with welding, and the adhesive itself will give protection to the seam. Only issue I've heard some people say was ghost line showing up in some cases when adhesive is used, There are many good panel adhesive manufacturers, some of the more common ones are lord fusor, 3m duramix , sem and usc. 18 gauge I would think wouild be a fine gauate for a patch. I use about that gauge for older vehicles, but it is a little more difficult to form by hand then 20 guage. Clamping gives the adhesive it strength so it should somehow be clamped or screwed in place while the adhesive cures. Both piece should be clean and ground. Lord.com has some repair procedures on there sight I believe if you look around. Getting the adhesive hot by welding to close (probably won't apply, but just as a warning) to the seam once cured I believe will comprimise the joint, this is how panels are removed by heating and softening glue with a heat gun. Another thing is you need a special gun to apply, and they are fairly pricey for a one time use, unless the place you buy adhesive from has a loaner gun. Again not your case, but when replacing large panels with adhesive, as far as I am aware, they still recommend placing a few spot welds at the front and rear areas of the panel.