I've patched both ways, welding and adhesives, both work quite well when done properly. When done with the patch, I always coat both sides with epoxy primer, then a coat of seam sealer around both the open and welded joints to prevent water wicking in to the joint.... On a patch that I want a smooth finish, after the epoxy primer I then grind the welds and use a skim coat of 'glass impregnated filler from Evercoat, it is waterproof and won't allow water into the joint. Whether seam sealer or filler, make sure you apply another coat of primer, then some paint over the patched area to prevent rust.